Let's talk about our favorite aspect(s) of each gen's main game.

R/B
Searching for the legendary birds was a fun sidequest.
The safari zone was (IMO) the best in the series.

G/S
The music and graphics always appealed to me.

R/S
The huge variety of ecosystems in Hoenn.
OST was memorable, at least once you hit Slateport.
Regi sidequest.

D/P
Also a great OST. Sinnoh in general was always my favorite, with the graphic style and various locations. The large amount of legendaries made for a fun post-game.

B/W
The variety of great Pokemon.
Some of the best tracks in the game.

X/Y
Very fun playthroughs every time.