Definitely Gen 5, B/W are great by themselves but when you add the only actual sequels we ever got in B2/W2 it is easy to see that Gen 5 is better than any other generation.
The second place is Gen 3 for me because R/S/E were great plus we got the first and the best Kanto remakes in FR/LG (my favorite remakes so far).
Third place is Gen 4 and it would be above Gen 3 if the D/P, and even Platinum in some ways, weren't so slow and sluggish and on top of their slow pace they have a bunch of annoying and slow Routes and areas that slow the game down even more (swamp and snow areas). HG/SS are the best looking remakes but they fail to fix the terrible level scaling of the originals (and that ruins the experience a little bit) and they heavily rely on gimmicks like following Pokemon and even Kanto post-game (but Kanto stuff is more the Gen 2 and G/S/C fault but it's still a part of HG/SS too so...) to make them a great game. Without these "gimmicks" Johto main story, despite the fact that I love this region so much, would just be to short and lackluster to make these games good without the extra stuff they added unlike for example Gen 5 where you don't even see a half of the region until the post-game but you still have more main story content than the entire Johto region ever had. The "idea and concept" so to say of all the Gen 4 games was amazing (probably the best ever) but the technical execution wasn't very good, however D/P/Pt and HG/SS are still great games despite all their flaws, just not good enough for Gen 4 to beat Gen 3 and especially Gen 5 as the best generation ever.