Well, my strategy is Meowth. Here's why.
Pickup ability, if you don't equip an item, you get a free item of random sort every floor. Immensely helpful, since its usually an edible of some sort, like the dungeons gummi or apples.
Need Money? Meowth learns Pay Day, when you defeat enemies using this attack, they turn into money! Fear not, they can still join you after they've been coined.
You can also learn Faint Attack by leveling up, 100% accuracy and owns ghost types, and I personally prefer Shock Wave from a TM. Sure hit electric attack, fries the legendary birds from a distance!
I recommend Mudkip as a partner, as he smites electric types with mud slap and can learn surf and dive, which means you don't have to train up a new water type.
It's up to you though, items and riches, or something else. It's up to you in the end.