Here is a simple guide based on my experience for those who are wondering who is good and who they should focus on. This is by no means comprehensive nor am I a top player, but it will help to give you an idea of who to focus on gathering fragments for.
Starters:
Ryker: Ryker is a solid dude. He can be a bit squishy for mid line, but he has good dps and since you can collect his frags in hard mode hes a decent choice. However, he typically gets outclasses in stats and abilities as some other dps guys out there so feel free to drop him once you unlock something like clyde.
Butter: Another solid guy. Butter is a very reliable and traditional tank. He taunts, he shields the team, and he has decent dps on his basic attack. Hes definitely one to consider lvling up as you go and keeping him around. Hes harder to collect frags for, but since hes a 3 star starter, you get a decent guy out of box.
Nightingale: A sniper healer combo always sounds great, but because shes at opposite ends of the spectrum (dps and support) it wears her thin. She doesnt do a ton of damage, not a ton of healing, and shes fairly squishy; especially since shes a natural 1 star. Once you get some decent healing options I would drop her.
Early guys you can get:
Cross: This guy seems like he has potential, but given that he is a natural 1 star, its hard to justify his spot in higher end teams. Early on, he will be useful just to fill an empty slot, but hes nothing too special. Decent DPS if player controlled, weak abilities (enemies can move from aoe and his stun is small) make him hard to use long term. He does have a seemingly useful platinum ability to just continually the whole team, but bringing him all the way to platinum seems risky of an investment. Might be an interesting side project for a second or third team when fully leveled, but definitely one to shelf for a while.
Sentry: Previously I said this guy was terrible, but hes not really the worst. As a straight DPS sniper hes OK. Hes not one I would invest too much into (and thats prettty much the same for all Krutz guys), but hes a decent guy to use for a bit to fill a role or a second/third string team. Simply put he sacrifices nightingale flexibility of DPS and support with a bit more DPS. Someone like Moss outclasses both.
Semi-early game guys:
Matador: This guy rocks. While he may not totally replace butter as a front line guy in the sense of taunting, he can replace nightingale as a healer (with maybe another support healer). Definitely one to shoot for to get daily frags and from pvp.
Dogface: As the best reward from gauntlet, hes not a bad one to pick up. I havent used him or needed him much, but I hear hes great for pve so definitely worth getting if you have trouble with story missions.
Beck: a decent DPSer. Pure stats for her are actually really good for a 2 star and since she has farmable frags shes a solid one to focus on for a dps class. She can be squishy like Clyde and I often have to control her or the AI will kill her in 2 seconds, but shes decent overall.
Keel: At first Keel seems like nothing special, but once you unlock her gold ability she becomes insanely good. Many consider her a top healer simply from this ability and really shes no joke. She requires some time investment to get to gold, but its worth it (unless the devs nerf her).
Salvatore: Unlock through dailies completion. I have not used this guy nor do I seem him used much or talked about. Makes me think he is not too crucial, but if I think differently after using him ill update this.
Cast: I think this guy used to be good as i saw him talked about on forums, but I think he has been toned down or nerfed. From my experience hes not that great. Hes a front liner who plays like a stunning mid liner and thats not exactly ideal. Not one i would spend much time on.
Hardscope: Never see this guy used and I havent used him either. Maybe because I have better guys, but he really doesnt seem very good.
Other heroes you can get from pvp:
Sentry: again, only decent and a considering that their are much better guys in the pvp store, i would not waste my time unless all the others listed below are like 4 or 5 stars already.
Maven: Havent used her much, but have heard she is quite good. Specifically her 3rd ability passive seems nice and her second ability to lock someone down for 10 seconds (a loooong time) seems quite useful.
Pris: Pris seems pretty good. I dont think she does anything special that you cant get from others so shes not a must or top of her class, but shes not too bad either. I would focus on a few others in pvp first such as maven, clyde, mandrake, or matador but shes pretty good.
Matador: as said, this dude is great. focusing on shards for him to get him the 3 or 4 star early on is good if he becomes a staple of your team.
Clyde: Dude is dps. He can put out awesome damage, but can sometimes be risky with his abilities and the fact that he is paper thin. I know when i play against him in pvp i focus him down fast. However, hes still a solid choice if you need dps.
Mandrake: A really really good support class guy. He does a lot and he does it well. First off he is a frontline guy so he can draw fire. Secondly he has a nice stun bronze ability that can break cover. And his gold ability is nice as a support/clutch healer. All around a solid guy for just about any team.
Richter: This guy is …interesting. He seems like a decent guy; front line guy who counters other beefy guys. However, hes dps seems low, low fire rate, and just ok abilities. Hes not bad, but i havent found a team or a way for him to shine yet. Not one i would personally invest in but when you just need a decent frontline guy to take cover, hes ok.
Gammond: This is not someone I have, but I have heard he is quite good. He essentially has taunt since people dont want him to stick around as the last guy and with his second ability getting cooldown breaks he can be pretty formidable it seems. Ive played against him in pvp and hes tough to kill/deal with. A solid choice and one i plan to work towards.
Others I have from crates I can comment on:
Baron: An interesting frontliner. Hes a huge clip and decent abilities. Hes designed to mow down cover so he can par well with others who benefit from cover breaks (hint hint Keel) but ultimately feels like he needs a special team to be effective. Still seems a bit squishy for a frontliner, but maybe with another taunter and good healing support he can work.
Halo: Kind of like a better nightingale. She is a rear line dps/support type. She has a nice big team heal, but it needs to be timed well and that can be tricky. She also doesn’t have a crazy easy/fast fire rate so she takes a bit of time to get used to. I typically use her in paring with another healer (like matador) so they cover each other. And since the timing of her heal isnt as straight forward, the AI actually does an ok job of managing it.
Heimlock: Largely considered the best healer in game. With solid stats since hes a nat5 and 2 team wide healing abilities hes very effective. The only issues are his small clips, slowish reload time, and long cooldowns. These are small issues though when he can do a lot to keep the team up and you can almost flip flop between his 2 team heals to keep the team up most times.
Francoise: On paper she seems quite good. She can heal herself, heal team members, can boost other stats, has a fast fire rate and quick reload - all good stuff. However, her dps is low, her silver ability takes a long time to power up fully, and her healing is subpar compared to other key healers (and no group heal). So really she cant cover dps on her own, she cant cover all healing on her own, and she doesnt give enough team wide buffs to be considered a key support class. Shes honestly spread too thin and doesnt have a defined role worthy enough of a high level team; for a natural 5 star thats pretty bad. Hope she gets some buffs cuz right now, i really dont think shes worth the time.
Phoenix: An decent DPS hero. Phoenix has decent abilities and has a good size clip with decent damage. She tends to shine at finishing off enemies at around half health with her silver ability. Enemy AI tends to run away when their health gets to like 35% or less, but since her ability locks onto them, they got no where to run. The downfall of Phoenix is that her health isnt exactly great and with her silver ability, she can slowly kill herself. However, with a decent healer and the chance to revive with her gold ability, she can do quite well. She may not out DPS the likes of Odachi or Hideo, but shes pretty good.
Ronin: One of the most balanced characters I have played with. Decent health, armor, fire rate/reload, and DPS. Ronin is a solid addition to any team and can have interesting DPS spikes with his silver ability. Hes often one of those guys who can keep pressure without drawing too much attention and he tends to be one of the last guys killed in my teams. Maybe that makes him a bit weaker in DPS, but overall hes not a bad choice. I like to use him when i have my healer, tank, and dps roles defined on a team and just need someone extra. Hes almost like a better version of Francoise as he plays a broad roll in most teams, but does it well as opposed to her sucking.
Other I have, but have not focused on yet are: Phalanx, Bolt, Operator. Let me know if you want updates on those once I get time to play with them a bit. I also want to do a more indepth review covering clip size, reload time, and fire rate. These play more of a role than I think people realize since we all just see abilities in the hero section. However once you start to use guys you get a feel for their overall design and approach when the basic attack is included in the mix. I also think its important to consider how well the AI controls each; i mean…you can only use one dude at a time 
EDIT: one item i forgot to mention; given that this game has 3 element types, some lesser guys can trump better ones for certain match ups. An example might be using Cross instead of Ryker for a given pve level or if your pvp team has no chem guys. Team composition is important. This list is just to give people an idea who to focus on since it can be overwhelming at first.
EDIT 2: Updated a few guys and added a few new ones as I got to spend more time on them.
Let me know any you guys have used and recommend/hate. Happy hunting!
