My favorite character. Wow. That is a loaded question. I don't have one favorite, but many that are tied.
Soundwave, Shockwave, Starscream, Megatron, Bruticus, or basically just the Combaticons , Astrotrain, Cyclonus, Rumble, Lockdown, Tarn, Skywarp, etc.
Optimus Prime, Rodimus Prime, Ironhide, Prowl, Sideswipe, Jazz, Jetfire, Metroplex, Superion and the Aerialbots, Grimlock, Ratchet, Bulkhead, Blurr, etc.
Now, favorite toy is a bit easier. I never got many as a child, but out of the ones I did, my favorite is the G1 Rodimus Prime my uncle gave me.
Animated Shockwave, Grimlock, FoC Grimlock, CW Superion, MP-13, MP-12, MP-7, and MP-6 are just a few I really want.