I'd like to second that comment on Trollirisu. It softened up many a 'mon in my reborn run with moves like charm, nuzzle, superfang, sweet kiss, and electroball. The pokemon I thought of may not necessarily be underrated, but I was certainly surprised how helpful it was in Rejuvenation.
This little guy just wall-stalled many opponents in my run. Its BST is only 410, but it has 130 Defense and Sp. Def and though its HP is only 55, it comes with lots of level up moves to mess with people: Pain split, Recover, Toxic, Baton pass, Counter, Safeguard, Taunt, Gastro Acid, Soak, Curse, to name a handful. On top of that, by TM it can learn Reflect, Light Screen, Rest, and Protect. Its abilities Innards Out and Unaware can both be helpful, and you can load it up with leftovers or a sitrus berry to up its tankage. There were some gyms where this guy took out most of the team by himself. Definitely turned me into a fan.