Once you get a character worth leveling (Xforce, 4hor, Nick Fury, Thor, Patch, Daken, CMags, Deadpool, Blade, BP, Hood, Captain America, Loki) up to 13 covers, it is best to level them as fast as possible. Any ISO you get that isn’t reserved for skipping or boosts should go into that character.
That character will, from that point on, be the cornerstone of your team.
From that point, you have two options: build around that cornerstone, or develop an alternative cornerstone to build around.
For example, I first maxed Hulk. So I needed to find someone who worked well with Hulk. The next character I maxed was Daken, because he had very good synergy with Hulk.
Then I maxed Patch, as he had synergy with both Daken and Hulk. Then I maxed Thor because he fit with Daken and Hulk as well.
And then Sentry became the new craze, so I began leveling him up. He became a second cornerstone. I was no longer building around Daken and Hulk. Now I was building around Sentry.
So that is when I leveled Hood and began leveling Xforce.
Then Sentry was removed from the game, so I continued to level Xforce to become my next cornerstone. Around him, I built Deadpool and hope to begin building 4hor soon.
Don’t just max a character because you have covers for him. I did this with Falcon and Human Torch and have barely used either since. Make sure the character you’re leveling matches your playstyle and your existing roster (or if you plan on making that character a new cornerstone) before spending ISO on them. Once you get into the 3 star transition, ISO is worth almost as much as HP.
Only boost featured characters if you don’t currently have a character with 13 covers worth leveling and it doesn’t look like you will for a while.
And even then…Maybe don’t waste the ISO. If your core roster is strong enough, you can score high even with a level 60 stock character on your team.