Choosing which heroes will get which crests isn't such a simple decision. Each crest type gives different stat bonuses, so you can't decide who gets what crest by talent ability alone. I will try to break down who gets what crest, and whether or not you should bother upgrading the crest - spending valuable red crystals.
Revive: useful at level 1 on any hero needing a nice secondary talent. Also great to level up for added % HP upon revival, and for the insane HP boost from crest set. May be one of the best choices to upgrade first.
Heavy Blow: useful at level 1 for DPS heroes like SZ and Ninja, or for the ATK boost for Druid's heal - even without a set. Great to level up to lvl 5 for well rounded ATK and HP boost, but I wouldn't prioritize HB over many other talents, depending on red shard income.
Revitalize: useful at level 1 for well rounded boosts in general. Doesn't need to be a set. Worth leveling up to level 5 for both the stat boosts and the ability. One of the top priorities if you have long-term focus since the actual ability isn't much use until level 4-5.
Scorch: bad crest for boosts; not worth leveling beyond 2. I'd suggest leaving it at level 1 or 2 simply for the talent's ability. Only use them as a set; scorch can actually be a nice talent. Sell excess scorch's for blue crystals.
Flame Guard: good talent, decent HP boost. Very useful as a set at level 1-2. I would suggest leveling this up to 4-5 if you have excess red crystal (after revit, revive, zerk, etc (don't really see that happening any time soon)). This should be the last crest on your list to upgrade.
-Heroes: Paladin, AC, Moltanica, Reaper, and most others.
Blade Shell: the troll's talent. I wouldn't keep this for stat boosts or talent ability. Only useful if you have no other crests. Sell for blue crystal.
Scatter: nice talent at level 4-5, but very costly to reach this high. Surprisingly decent stat boosts at higher levels. I'd hold on to all scatters in case you want to upgrade them in the future. Not really worth using at levels 1-3, even for stat boosts. I'd use it if you have no better crests to use, but it doesn't shine until 4+ just like zerk.
-Heroes: Aries, Moltanica, Reaper, Druid, and most others.
Slowdown: bad for stat boosts, not good unless a set. BEST DPS IN GAME. Very useful for DPS in mesa and boss. I highly suggest keeping a bunch of lvl 1 sets simply for max DPS in mesa. May also be useful to try "freezing" the boss. Also a decent talent for HBM. I wouldn't bother taking up beyond level 2.
-Heroes: All (depending on the scenario).
Sprint: awesome talent as a set - weak crest for stat boosts. I would still advise holding sprints and possibly leveling up to 3 for a short term benefit. Also very good for HBM heroes. May be worth investing in for long-term use - especially in Arena. I'd definitely spend crystals to level up to 4-5 if I had enough crests.
-Heroes: Druid, Aries, PD, Ninja, and most others.
Berserk: amazing talent when it reaches level 4+. Fantastic ATK boost, even without a set. Probably one of the best talents to place on Druid for the heal boost. I would hold these crests until you have lots of them while spending red crystals elsewhere in the meantime, or gradually level up and place on a healer/DPS. Sadly, you won't see much benefit from the actual talent until 4+, so I think focusing on other talents like sprint and bulwark is smarter.
-Heroes: All except Ninja.
Stoneskin: decent HP stat boost when coupled with the talent's ability. I wouldn't suggest bothering with this talent except for keeping a few sets early-on until you get better crests. Similar to FG except it has no offensive power (reflecting damage).
-Heroes: Paladin, Minotaur, Orksbane, Moltanica.
War God: awesome ATK boost and adds ATK %. Great on Druid and other DPS heroes such as SM or Ninja or Vlad. I would suggest leveling this up as you get more crests.
Tenacity: poor stats, but ok talent ability at level 1. I would only use as a set at level 1 and sell extra crests for blue crystals.
-Heroes: Paladin, Orksbane, AC; none.
Life Drain: one of the best crests at level 5 for talent ability, but only offers a slight boost in HP/ATK - comparable to stone skin. Not very useful as a stat boost, yet great secondary talent. I wouldn't make this a priority, but I would suggest holding these for some day in the future.
Bulwark: nice overall talent for both stat boost and talent ability. I would say this is probably the most balanced crest to level up as you collect them, considering the ease of obtaining bulwark's when compared to Revit and Revive. Definitely keep all these. Useful for nearly any hero. I'd make this #1 priority for upgrading until you start collecting revit, revive, berserk, and life drains.
It seems, as expected, revitalize, berserk, and revive are the best crests. I'd rank bulwark, scatter, life drain, war god, and heavy blow as the tier 2 crests. Sprint and Flame guard are the only talents I consider tier 3. Scorch, stoneskin, slowdown, and tenacity are tier 4. That leaves only self destruct, deadly strike, and blade shell as tier 5.
Order based on set stat boosts plus talent utility:
Tier 1: Revitalize, Berserk, Revive
Tier 2: Bulwark, Scatter, Life Drain, War God, Heavy Blow
In the table attached, I have the crest data for a level 1, slot I stat boost for each talent. I then found the average and standard deviation of each stat boost and came up with a z-score. The z-score tells us how "good" or "bad" the boost is compared to all other talents. Green means above average, red means below.
As you can see, the Revive HP boost really is as awesome as it seems. It's 2.7 standard deviations from the norm for the other talents, head and shoulders above the rest. Ditto for zerk and attack boost.
The last column, Combo_z, is the sum of the HP and Attack boost z-scores. It's a balanced look at how good a crest is for both HP and attack. Here, we can see that even though the HP boosts of Scorch, SS, LD, FG, and Tenacity are good, the Attack boosts are so low compared to other crests that they're slightly below average. Revive's amazing HP boost compensates for its super low attack boost. Notably, Revite is the only talent that's above average for both HP and Attack. BS and Sprint are the only talents below average for both. I think this table is handy because you can see at a glance what a talent is good for (HP or Attack) and how good it is for that boost compared to other crests. Lastly, it's important to remember that as a set the talent might be worth it even if the boost isn't great. Sprint is a nice talent as a set even if the stat boost is second-worst in class.
EDIT: This is a fascinating dataset. I'm looking at the other slots II-IV (still level 1) and finding some interesting things. Will report more soon.