I have played/owned:
Leafgreen
Emerald
Diamond/Pearl/Platinum
Heartgold/Soulsilver
Black/White
I still have to play B/W2 and I haven't even finished B/W yet.
I have and played the first two Pokémon Ranger games and I'm halfway through Almia Signs, It's hard though, since they are so storybased with endless speaches, that are so childish and cliché.
The Pokémon Mystery Dungeon series are my favorite, maybe even better than the regular :x
I love the story, plot twists, love- and hate-able characters and the way you explore the dungeons.
I haven't play Gates to Infinity and tbh I don't think it will be as good as the previous ones, but I just picked up a 3DS (Pikachu edition <3) so I will buy it eventually.
Pokémon Conquest was an interesting concept. I started playing and loved the gameplay immediately, the story not so much. But halfway through I got bored and quit, I will have to pick it up again soon.
I haven't played the Colosseum/XD or any other Pokémon games.