I'm new to FE 6 but I've completed just one hard mode run, so take what I share with a pinch of salt. Geese is arguably my fave, since he's really good at taking out enemy dragonmasters, especially those from the southwest corner in Ch 21. Just give him 5 killer axes and warp him into the mountains and watch the fireworks. The mountain's +40 avoid is insane - the enemies will pretty much have 0 hit vs Geese. As long as nothing else is within those dragonmasters' attack range, they'll just orbit Geese forever. Gonzales hits harder and is faster, but I wasn't able to level him because he keeps missing his kills. While Geese's skill isn't spectacular either, it's enough to get the job done, and he can destroy some paladins too.
Rutger and Clarine are pretty much a must, though I wasn't able to use Dieck since he got badly RNG screwed. Even without Dieck's support, they're still powerhouses, and they still dodge like a champ. So I guess Dieck can be dropped. Fir replaced my Dieck, and became even more insane than Rutger with Shin support. Give Fir a chance. If she gets there, she becomes a godly weapon of mass destruction. If she doesn't, she'll still probably kick plenty of ass with that sweet critical of hers.
I preferred Lilina over Lugh in my run because Lugh uses up tomes so quickly. Also because Lilina is awesome vs bosses. Make sure to stock up on tomes for Lugh!