My favorite is definitely Volcarona. I love both Fire-types and Bug-types, and it's a beast in battle. Its design is nice, too- and it's a lot of fun to draw! Other favorites of mine are Galvantula, Kyurem, Scolipede, Genesect, and Sigilyph.
If I had to choose a least favorite, I'd have to say either the 3 elemental monkeys or Sawk/Throh. I'm not a huge fan of monkeys to begin with, and I've never had good luck with them in battle. Sawk and Throh are kind of good, but I'm not a fan of their designs at all.