In Red/Blue: I like Ivysaur's sitting up pose- it's a shame they didn't do this in any more sprites afterwards. Fearow, Meowth, Rapidash, Seadra and Moltres (with its unusual style) are quite lovely, for such old sprites. And Golbat is getting an honourable mention because... well... it's certainly one of the more memorable sprites. If only for how outright bizarre it looks.
In Green: Pidgeot is getting a mention, deal with it. I happen to like its pose- the R/B one just looks like one of those wooden bird things that you often see tipping into a glass of lemonade. Ninetales is absolutely gorgeous and by far my favourite Green sprite. Fearow, Ponyta, Rapidash, Tauros, Jolteon, Flareon and Aerodactyl are all quite nice, too. And Mew does look a tad like some sort of alien creature, but the pose is interesting. XD
In Yellow: There was a serious step up in sprite quality. Everything looks pretty good, for the graphics at the time. Ninetales, Persian, Rapidash (for a third time- it just has some really nice poses and sprites!), Dewgong, Scyther, Lapras and Articuno are some of my personal favourites. Although I think my favourite overall is Dragonite- it just looks so determined!