It's going to depend on a lot of things.
Paladin/General vs. Great Knight, Great Knight has three weapon types but both of the other class's weaknesses, I personally like the other classes skills. I don't know if you really ever need all three weapons but I guess it would be nice to have, my Kellam has the General's skills and is now a Great Knight, mostly for the extra movement, and his def is so high that he doesn't need to worry about anything that isn't magic.
Swordmaster vs. Assassin, for the main game I would take Swordmaster over Assassin because Astra's skill/2 >>>> Lethality's skill/4. Pass could be nice though as well as bows, so you could probably make an Assassin work easily enough. Postgame, Swordmaster has better Mag, Def, and Res caps, Assassin has better Str and Skl, Assassin would probably be better because of the added bows.
Assassin vs. Trickster, depends on if you want the character to be healing.
Wyvern Lord vs. Griffon Rider, Wyvern Lord has lances which is nice, but Griffon Rider has Deliverer which is really nice to have. Their Breaker skills are great to pass on to other classes, Wyvern Lord can make better use of its Swordbreaker though.
Sorcerer vs. Dark Knight, Dark magic is apparently stupidly good in this game, so probably Sorcerer.
War Cleric/Monk will need good Str and Mag, so most magic based characters would probably want tomes over axes.
There's a lot of classes I haven't really used, I'm so sad that I didn't use a Hero, they are usually one of my favorite classes, but no one started off strong enough to make it onto my team.
Edit: I'm going to disagree with Ussi on Falcon Knight vs. Dark Flier, Galeforce is always useful, being able to take out two enemies with one character in a game where your army's size is always going to be cut almost in half at the start is always useful. Flying magic is also amazing and there are enough other physical based classes out there when magic is so good, unless the character doesn't have good magic(like Cordelia) I would always choose Dark Flier.