Gible, Bonsly, and Buneary are the three that most stand out to me. I have a Sudowoodo but have yet to clone or transfer it from SoulSilver, and would still have to breed for Bonsly, so that seems easiest; meanwhile, Buneary isn't in the original Black (it's a Castelia City trade in Black 2 it appears) and I'm sure I'll find it in Sinnoh, but I'll have to complete it first to trade methinks; Gible meanwhile just isn't around (it's only found in a secret area even in D/P/P, and the one in Black 2 is Shiny; again, I'll find the secret area eventually, but that doesn't help now ;) ).
If you still want the Shiny Gible, I can clone and trade it back for junk. Your choice. :) Kanto is complete as of this morning (just three legendaries, and all found in SoulSilver) and Unova already completed months ago (only Meloetta remains), and the Johto exclusive group is very small (already have the ones there for Black 2, same with Hoenn's batch, even Spoink which arrived via trade). Even have an Egg Move Magby (DynamicPunch, which I've tried out, but 50% accuracy stinks).
So yeah. Bonsly, Buneary, and Gible. :)