Best: FE 12 Feena. I have not played this game yet, but I'm voting for her nevertheless. The only dancer that can attack, has great growths. Not only can she support with dancing, you don't have to worry about her being a liability on the field due to her being able to defend herself. Great.
Worst: FE9 Sothe. He got insta-killed by a Bolting. Even though he's free, he's a major hassel. Especially with Blossom not allowing him to gain anything. And if you take off Blossom, you're kind screwed in Resistance. Though I believe every unit can be great when leveled, Sothe has some seriously restrictions: Not being able to steal anything from people with a higher speed than 20, not being able to steal weapons that weigh more than 20, not being able to forge weapons for him... It's a recipe for diaster. And personally, I like Sothe.
I'm uncurably biased, but sometimes you have to look at things differently than they initially appear. In defense of the Sisters...
How is Nyna the worst? When compared to the rest of the 4 Sisters, actually pretty good. That Luck alone explains how she survived Ch 20 of Shadow Dragon. And she comes in with a Fortify and an A in staves so she can use all staves (except the Hammerne). Her growths are good, so she's a great a support unit, and with some steriods she becomes an able fighter.
Clearly I'm biased, but in defense of Maria, how can she be a redundant Boah or Elice when she joins before them? If anything Elice is a redundant Maria as her main aspect (besides her awesome growths) is using the Aum Staff that Maria can use, and if she was used then she's already at a much higher level. And Wendell is most likely surpassed by a trained Maria.
Anyone. Seriously. As if Matthis is going to make a better 'cleric' than Maria. Gordin, Roshea, Vyland, etc - none of them are going to make a better 'cleric' in terms of staff utility and support.
Actually, in the first few she has a great Weapon Level and growth in that area, combined with good defensive stats, (both better than Lena's), so as a support unit, as she easily gets weapon level and can use many staves easily, she is extremely able, especially at a distance with Physics.