It's simple - why not? Randall is arguably the best animated villian - not just by Disney - but of all animated movies I've either seen or heard of. I loved the complexity and detail of the computer animation used to create such an intriguing character.
His voice was another point that I adored in him - it fit perfectly! I couldn't of found a more fitting voice talent to bring this lizard monster to life. His personality shined in the movie as well, he's the kind of character who steals the spotlight - villian or not. I really don't think he's as nasty as he makes himself out to be. ~_^ And because I'm not the kind of person who'd love bad guys of any type, the fact that Randall found a place as my favorite character of all time is a pretty big feat.
Like his voice talent, Steve Buscemi, I loved the fact that he had so many limbs. It was a very unique and imaginative trait (it also makes drawing him fun). Another example of a really awesome trait is his blending-to-invisible ability.
Overall, you can figure that Randall rules! Why else would I dedicate a large site site to such a talented character?