Seconding Sprung's mention of Grumpig! She really already hit on most of what makes Grumpig good, but I'll also point out that in old Hoenn specifically, the pure Psychic type is excellent defensively as well when it combines with Grumpig's stats as a special wall. Its only three weaknesses are either Special (Dark, exacerbated by many Dark types being physical attackers), rare (Ghost), or terrible offensively (Bug -- three solid moves are learned by a combined six pokemon).
In my own recs, gotta point to Leavanny. I can easily understand why one would hesitate to use this guy -- after all, Bug/Grass is an awful defensive type, with six weaknesses and two of them being 4x. That said though, Flying and Fire coverage is very rare in Unova, and Leavanny's 75/80/70 bulk is surprisingly solid for an early-game bug, so if you do face a coverage move of one of the other four weaknesses, you'll probably be fine.
But that's not where Leavanny shines. It's in offense. Leavanny gets Bug Bite and Razor Leaf pretty much immediately in BW1, and even in BW2 you'll have the former before the first Gym. Later levels bring it Slash, Leaf Blade, and X-Scissor for much-needed upgrades that still fit Leavanny's excellent 103 Attack and 92 Speed. And of course, at level 46 (endgame in BW1, just before Drayden in BW2), you get Swords Dance to amplify the physical power. In addition, TMs give it some further coverage as well in Return, Aerial Ace, and Shadow Claw, plus Poison Jab in BW1 if you really hate Grass types for some reason.