Not counting the remakes, these are probably my favorite games in the series, albeit they do have a lot of issues. To summarize:
+ Kalos was a well designed region (although it was a geographical regression after how exotic Hoenn and Sinnoh were)
+ The newer Pokemon were all consistently good
+ The graphics were gorgeous
+ The battles moved fast
+ Mega evolutions and the new type up chart completely changed the dynamic of the game
+ The Super Training, Pokemon Amie, and Player Search System, were all great additions; the latter especially breathed new life into the series, making it a persistently connected multiplayer game
+ The music was great, although this does represent the weakest entry in the series.
+ Customization options were great
- Lumiose was a mess, as was the camera control
- The story was laughably bad, and it stuck out like a sore thumb after how good the Black/White games were
- The game was too easy. There was literally no challenge, and as a result, no sense of progression or accomplishment. This, perhaps more than anything else, hurts the game
- The level curve is completely broken, a problem compounded bythe EXP Share
- Too many technical issues- no 3D, framerate that resembles a slideshow, bugs, glitches, loading screens, you name it. This is something that needs a lot of work.
- Absolute lack of a post game kills the lasting value of the game, and is a shocking regression (why, for example, is there no Elite Four Second Challenge?)
- As mentioned above, it was the weakest soundtrack in the series.
If I had to rank the games, here is how I would do it:
- Pokemon HeartGold/SoulSilver
- Pokemon FireRed/LeafGreen
- Pokemon X/Y
- Pokemon Black/White
- Pokemon Ruby/Sapphire
- Pokemon Diamond/Pearl
- Pokemon Black 2/White 2