This is a very difficult question.
As a long term player, I would put the latest three generations above the earlier three, simply for the improvements in gameplay that happened in gen 4. The physical/special split is huge in terms of making a whole bunch of pokémon more viable. That plus the huge amount of new moves (or at least that's how it felt) that capitalized on the split. Finally a bunch of mons were more viable - mainly the physical dark- and psycic types. I feel that the split just made the game more fun.
Also, I
As others have pointed out, the story of BW/2 is probably the strongest in the series (ymmw ofc.)
The reusable tms and the lowered focus on HMs to hinder progression through the game was a nice change.
What I didn't like in BW (but was sort of amended in bw2) was the low amount of available pokemon in each area, especially during the beginning of the game. To compare with gen 1, where you could encounter (disregarding starters here) 7-8 different families before the first gym, in bw there were like four, including the monkey.
The diversity sort of kicks off in the mid- to late game, with some great mons like Siglyph, braviary, mandibuzz, the threestage bugs, axew and hydreigon (although you probably won't be able to use one before the aftergame due to high evolution level) etc.
The beginning of bw always leaves me sort of disappointed.
Also the introduction of pick-your-own-elite4-member! Where you can take them on in whatever order you so choose. That's a great improvement in my opinion. It prevents you from having to level in order to be able to take on the champion or the later e4 members - and in the process making the earlier ones a complete joke.
I'm playing through HG currently. I was quite a bit under Lance pokemons levels, but way too powerful for Will and Koga to be a challenge at all.
XY sort of continues the path that bw put down, but since it includes old mons as well as new in the main game it has that typediversity that bw lacked. I thought it was just a superfun game to play. The introduction of hordes and the friend safari (and the pokefinder thing in oras later) made finding pokemon with hidden (and fun) abilities much easier, and the supertraining allowed all to monitor the EV progress and all that. Also, pokemon amie is adorable

The one bad thing about XY really is that it really isn't a difficult game. Especially the later gym leaders and the elite four really has too few pokemon. Three and four respectively? c'mon
