Pinsirius
Sentimental Fool ;)
I'd say Red or Blue, actually. Yellow was my first game, and albeit very challenging, as I was only five or six at the time. But when I went back recently and replayed the games, I found that the ability to obtain the three Kanto starters makes it a MUCH easier game, with less difficulty finding good type-matchups. Furthermore, Charizard gains the ability to Fly, meaning that one does not need to "waste" a space on their team for the rather weak movepool of Pidgeot or go through the tedious proccess of obtaining and training an Aerodactyl or the legendary birds. My latest go through Yellow was simpler than any other game I've gone through since.
Interestingly for me, Yellow had the opposite effect. The starters were helpful, but I was also reminded that none of them- particularly Charizard until he learns flamethrower- are top-class Pokemon. They helped with type match-ups, yes, but what is it worth when your Charmeleon/Charizard is embering Koga's Venonats firing back with Psychics?
Anyway, Yellow. Red and Blue's movesets were so bad I could beat the Elite 4 with Pokes in the mid-high 40s. You can do that in Yellow too, but movesets that are actually good goes a good way towards offsetting simplistic AI.