Does the Ultimate team ability work against characters in hindering terrain with stealth? I NEEDS TO KNOW!!!! lol

From the Marvel Team Abilities list:
Ultimates: Members of this team ignore the effects of hindering terrain on movement, combat, and powers and team abilities.

So we see that Ultimates ignores the effects of hindering terrain on Powers (Stealth) and Team Abilites (Batman). I'd say that your need has been met.