if we're talking pokemon
RBY obviously the start of the franchise and was awesome.
GSC were really big games and that was really great because not alot of games are this big anymore, along with the new pokemon that were added being pretty cool and unique to the original 151.
RSE had some good ideas for actually vsing other people with the addition of pokemon powers (although they really don't use this to the full capacity it could be except on shedinja) and more thought on the stats and stuff. probably has the nicest looking ubers in the series (cept M2) but god **** this game lacked an expansive world, espiecially considering it followed GSC... you could finish it in under a day. on the plus side it has set the standard for all new games in terms of how gameplay and stats are.
FR/LG were nice remakes but nothing too special
DP doesn't expand on how the existing system could be used, adds further imbalance by making single stat stacking even more desirable, and you get to play rock/paper/scissors with what item your opponents lead has equipped which can quickly lead to instant loss. giving crobat hypnosis = wtf nintendo. the DP pokemon are also pretty unimaginative and boring except for lucario because he's cool.
GSC was the best in terms of the overall game, but i do like the battling system RSE introduced.
Lucario for Brawl :3