Eirika is really easy to use, she's effective at her job with no setup. Simple, strong unit.
If you've got a team that can throw buffs around reliably (like using Fae), Marth can be a monster, but he'll still go down to strong blues. He's a bit more versatile than Eirika though.
Gatekeeper is a bit odd. His weapon effect is pretty good, but nothing to write home about.
Marianne is... on paper her effect is really good, but in practice I still haven't been able to apply it to any real advantage a single time. The limit in movement speed in the ally she "dances" and her reliance on landing two hits and getting hit once (which is often an issue because her attack is too high to let some enemies live long enough to hit back) just make her unreliable. She might be really good in AR, if you play that - I don't.