Favorite -- GSC. 2 regions, although Johto was small. The pokemon selection lasted me through MANY run throughs. I (only playing Gold and Crystal) probably ran through the games about 5 times in a year or so. It was so good. I haven't ran through it completely since though. The same reason the OP put up to, I probably never beat the original RBY versions. Maybe yellow, but as for Red version -- it probably never happened (never got blue version, bro got it and silver). I was still getting the hang of the game, and my strategy was to level up charmander and solo the whole game with him with a level 3 cateprie and pidgey at my waist for nothing. I didn't even know about the HM system that well.
My favorite Spin - off would have to be the Mystery Dungeon series or the Stadium series.
As for worse.. it's hard to say. Probably Battle Revolution. OMG GRAPHICS AND ONLINE RANDOM PLAY OMG. >.> It's like the Brawl version of the Stadium Series.. Stadium 2 being Melee, and the hardest. XD.
and as for non spin-off... it's probably DPP. I still like it a lot though.
The least interesting for me was Leaf Green... just a copy of the old versions but it was the version without Growlithe... lame! What's a game if I can't get my precrious ARCANINE?!?!?!?
That's why Ninetails is there.