It’s hard to write a guide on this because so much of it depends on the state of your 4* roster and whether or not you’re comfortable with the MMR and scaling changes that come with it.
Some people will say that even a useless covered 5* is worth leveling at a certain stage because of it’s health and match damage alone. I don’t agree with that personally but that doesn’t make me right.
Generally speaking:
OML - Viable at 1/1/1. His red passive is great at just one cover. At this stage he’s good for pve and playable in pvp. He won’t compete with buffed 4*'s though.
Phx - Viable at 1/3/3 or 5/1/3. 1/3/3 is probably the minimum where I’d be comfortable using her as a main in pvp and pve. You might get away with 1/2/3 or 1/3/2 but that’s a tough call especially if you don’t have OML helping with strikes. 5/1/3 is also viable especially if she tanks. You want her to die and respawn to win matches that drag on. This can be done with fewer than 5 green but it just adds to the risk factor so to each their own.
Note: A heavy purple Phx with OML red is viable. So 1/5/1 or 1/4/2 with a viable OML should work fine too.
Surfer - Never Viable - My view on Surfer is that you do not level him until you have 2 other viable 5*'s. You never want to be in a situation where your MMR involves your Surfer against everyone’s OML’s or Phx’s. Surfer can win but he’s not as strong and it puts you at a disadvantage. For PvE he’s viable at 1/1/5.
That’s all I’ve got from direct personal experience. The rest is simply opinion and could definitely be wrong.
Goblin - Viable at 1/3/1. At 0/3/1 Goblin’s purple steals and absorbs 27 AP when used on a purple tile. That alone makes him worthy of consideration on purple tiles. At 1/3/1 I suspect he might be viable for pvp as well.
Cap - Never Viable - Same problem as Surfer but even worse. Don’t level him until you’re comfortably in 5* tier.
IM - Viable at 2/1/1 - This is based on the condition you have a championed Rulk to pair him with. IM is the only other 5* whose strong early on. You might not even need that second red but I don’t have direct experience with him to confirm. Again, when he’s low covers like that then consider buffed 4*'s to be your superior option still.
Banner - Never Viable - Same as Surfer and Cap. He’s a support character. You shouldn’t push him until you’ve already entered 5* tier.
Black Bolt - Viable at 1/3/0 - This is purely based on how he looks on paper. At 3 black he’s acting as an AP generator and should definitely be viable in pve. Add one green and you have a damage finisher in PvP. In theory it works especially if you have something close to a rainbow team to pair him with.
Black Suit Spidey - Viable ??? - I’m at a complete loss with this one. Mine is 3/1/4 and it’s not remotely viable. I suspect he needs at least 5 blue and probably several green to be considered. This is a character that shines at 13 covers. He doesn’t seem to build up well.