I'm going to have to side with Ruby, Sapphire, and Emerald on this one.
Although they might be old and their remakes--Omega Ruby and Alpha Sapphire--are far better in terms of graphics, sound, and various other aspects, what makes me like these titles more is nostalgia factor; every time I play, they never cease to remind me something of the past, which I do like.
Aside from these three, I also like X and Y. They're not very good in terms of plot, at least in my opinion (to me, things seemed to have been a little rushed; would've preferred it if they'd taken it a little slower), but they did deliver when it came to mechanics and features (I'm positive not many of us could deny that).