Gold, Silver and Crystal definately. Reasons:
Day and night
Real time
Great storyline (kinda like the R/B/Y storyline but better, the Hoenn storyline was rubbish though)
More challenging than any of the others (or at least I though so)
Very long game
Never a dull moment
This and many more. Although the Kanto games come pretty close, the Hoenn games are way off though.
I'm finished here...
Special thanks to Yossy666 for the avatar and signature images.