My favorites fluctuate about by the month, and Poliwrath is a recent addition to my revolving door of faves, but he quickly shot his way up to the top.
I always thought he was cool as hell, like this big buff boxer frog, but using him in battle clinched it for me. I first gave him a shot because I needed a source of Water and Fighting type STAB on my team. With a Bulk Up SubPunch Waterfall set, Poliwrath proved to be ridiculously fun to use, and although he didn't always exactly dominate the field, it was always great using him. And when he did get going, he GOT GOING, and watching Poliwrath plow through an entire team after a lot of Bulk Ups and a lot of bad decisions on the foe's part is amazing. Plus, Poliwrath's Focus Punch animation is EXTREMELY satisfying.
Huge thanks to /vp/ for helping me get him off the simulator and onto my cartridge. Thirsty Mike (aka The Thirst) has been quenching the Maison like a champ.