Here are some Pokemon that got me through a majority of my playthrough ("underused" is subjective, but I hardly use these Pokemon myself)
Kricketune - It was the backbone of my team early in the game, and its Fury Cutter just massacred Florinia's team.
Numel/Camerupt - It got me through some tough scrapes, and did lots of damage when fighting Corey, Shelly, Serra, Aya, Cal and Titania.
Beheeyem - Normally I don't use this Pokemon because it's slow, but it's been extremely helpful when fighting Terra on the Glitch Field. Set up 2 Calm Minds and watch it clown.
Sandslash (ground) - It helped me in the middle part of my play though, especially against Kiki. Swords Dance with the Sand Rush ability on the ashen field makes it lethal.
Hippowdon - I considered this a slight upgrade from Sandslash. It was bulky and helped me against a lot of the bosses and gym leaders.
Heliolisk - I mainly used it to do damage quickly or pick off weak enemies. It decimated Amaria's team with its electric attacks and its Dry Skin ability.
Florges - I call it the "Dragon Slayer" and used it against any boss who used a Garchomp.
Sudowoodo / Magnezone - Sturdy ability + Counter/Mirror Coat = Cheap ways to get through boss fights, lol.
Those are the ones I can think of on the top of my head.