Most of the fully evolved have been really effective for me. I'm trying to stick to using the best links on the story characters (outside of using Dragonair on Oichi for a while because Jigglypuff was annoying me). Have particularly enjoyed Empoleon, Charizard, Infernape(wtf Fire Spin is actually good in this game), Lairon (but not Aggron), Vaporeon of what I've used so far as far as offensive power. Have been a little underwhelmed by some stuff I expected to be better like Aggron, Wigglytuff, Steelix, Quagsire, and sort of Metagross which weren't as good as I'd expected for fully evolveds. Everyone uses them in the story but I liked Gallade and Rhyperior a lot more after I changed their abilities once you get the chance to do that into Conqueror and Bodyguard, respectively. Bodyguard in general seems really good (especially for Rhyperior since it's doing nothing half the time otherwise), I was using a lot of Pokemon with common weaknesses during the campaign which could have been a problem but whenever they were in any danger I just sat Rhyperior next to them to absorb the would-be-super-effective hits for them, was pretty handy.
In general I'm finding the characters that have a reasonable chance at killing multiple targets in one hit are preferable right now to speed shit up, if I need to defend shit or siege weak castles after the main campaign is over I'm sending solo units a lot more so stuff like Infernape and Empoleon that can take a bunch of targets down quickly are really nice.
EDIT: Also, while it's not as strong as stuff like Infernape, I would suggest Carnivine to people earlyish on. You can get it basically immediately and it has all the little stuff that make Pokemon good in this game: it has decent stats all around, an attack that hits multiple targets, and it gets a huge mobility boost from its ability that lets it avoid most of the map gimmicks in the game. Levitate in general is really good.
