Im gonna go with Zigzagoon. It has a cute design and Pickup is a useful ability.
I picked Sentret, because in the early days of me playing Gold and Crystal (before I knew any better) I'd actually capture one, and use it until I had six better things
None of them stand out to me due to the fact that they all serve pretty much the same purpose in the games, so I'll choose Sentret. Both it and its evolution look adorable to me.
Zigzagoon. Because it's adorable as hell, and it has never let me down with its Pickup ability.
Excepting a long forgotten play through of LeafGreen, the rodents don't ever make my team. But overall, despite initially having a Sentret/Furret fill my SoulSilver early on (was replaced right before Goldenrod), the best is Raticate to me. No nostalgia value, per se, just super speed and its general attack strength given it being the starting rodent.