To whom should I use elemental frags that I earn from heroniums? If possible, don’t just pick one but please rank. Thanks.
Is it the hero…
Useful in Co-op
Useful in PvP
Fewest frags to evolution
Nearest to 10 stars (isn’t it like exchanging frags for frags?)
Lowest star (to balance lineup for PvP)
Easy to farm (common heroes)
Rare (Like Heimlock, Phalanx, Prophet, Artemis, Brogan and the gilded heroes)
ill have to say heckler, since hes in the pvp store…
When it comes to this, it all depends on what kind of player you want to be. Use them on PvP heroes if you want to be a PvP player. Build up heroes for Coop if you want to grind for hero frags, or others raids. If you want to earn more heronium, to buy other frags, build up Matador, Mavin, Surge, and Cinder. (Daily Rewards) I would probably stay away from building up the 7 stars. Not because there bad, but in the end, there just like any hero who is seven stars. I would definitely build up 5 stars though, for the most part, all of them are good. Building the lowest of your lineup isn’t a bad idea either. If you haven’t already done so, I would build all your weaker heroes. Get all of them all above Gold. You could also build up the bonus faction heroes for bounty, if you want to deal heavy damage. In the end though, you can do some many different things with heronium. It’s your choice. Hope this helps a bit. 
I personally, am investing in common heroes easy to farm in hard mode (Ricker, Nightingale, Cross, Beck and Dogface) and the daily heroes (Cinder, Matador, Surge and Maven) so I have enough daily heronium farming to evolve more useful heroes. Many players derate the value of silver chests but they are an excelente source of heronium if you have most of those heroes at 10 stars. The weakly heroes are also a good source of heronium (Kuno, Cast and Sentry). Hope this helps.
Its not always successful, but I try to guess the element of the next featured hero. That way I build him fast. The hero will help you in War and Bounty as they are bonused heroes. Lately, The new heroes have been replacing (Overall Power wise) the previously top heroes. I have found that this approach works best for me. I have 50, 10* heroes, so farming heronium isnt an issue for me. If you do not have a lot of 10* heroes, I would use it on the daily quest heroes, and start building heronium farms. Once you can start acquiring all of 1 or 2 elements of frags, you will be able to build heroes faster.
In the end, like J_A_D_O said, it depends on the type of player you are, on how you should build. I have been building around war and it has worked out for me so far.
If you need a reference for over all power of heroes, in game check out Skathi. Its a dev account.
Like redomega said.
Once u have 1 in 10*, by focusing frags to other original/default heroes, in few times/days or month u’ll have them in 10* also.
Plus, default heroes r easy/cheaper way to obtain in hard mode (10 stamina per 1 hero frag, or will be 50 heronium vonvert if u have them in 10*).
I pushed enough heros to 10* to insure I’d always be able to farm 9k heroism daily(to buy all 180 nonr universal frags daily) once I did that I focused on heros featured for the month to Boost event value. After 18 months of reliable heroism farming I’ve now 10* all my heros(only missing 1 gilded hero, striker)