I love the Ratchet & Clank series too (why the hell didn't I buy a PS3...), but no games really come to mind that compare to it... including on other systems. It's pretty zany with the absurd weapons in the game, so not too many games compete with it in that compartment, lol.
Although it's not for the Wii, I could certainly recommend the PS2 games Jak 2 and Jak 3. Although it's not AS crazy, it's a non-realistic 3rd person shooter with some kick ass weaponry, and a somewhat similar feel to the combat. I loved 'em both. I'm still surprised they weren't made by the same company.
Jak 1 might not be your cup of tea, though. It's not combat based and it's more of a platformer, there aren't any guns at all in the first one. They really made a 180 with Jak 2 and 3, and in a good way.