I go back and forth between Skullcruncher and Mindwipe as to who's my favorite. It should be Weirdwolf (Wolfwire), because he was my favorite as a kid. But the figure is just a bit lacking. I have Function X-02 Quadruple U, who is better overall than Hasbro's figure, except the terrible hand design -- they did this thing where his dewclaw swivels around to become his thumb. A good idea, but poorly executed, in my opinion. If I could combine aspects of QU and WW, then he'd be perfect.
Unrelatedly, if you're wanting some decent quality, cheap, beast bots to possibly mod into transtectors for the extra heads, you could do worse than the
Fight The Beast line. I have the Fox that I use as a turbo-fox for Mirage to hunt, but I'm considering buying some others to mod into bodies for the heads. I think
the elephant would make a great body for Loudmouth. I'm thinking the rhino for Crash Bash or Apeface, not sure on the rest. I'm already buying RiD figures to behead and turn into transtectors, but I think it'd be fun to have some beast transtectors, too, especially for the heads that are from, or based on, figures that were originally beasts.