Hmm. Well. I dont really know.
I am currently playing through a marathon, from Gen 1 to the newest gen, and I gotta say it wasn't the easiest thing. I hardly remember first and second gen (if I ever even played it, it'd be almost a decade ago), so that was easy, although I did run through the Crystal so I dont spoil Soulsilver too much. Firered was also not that hard to replay cos I played it a loong time ago. Emerald was a bit (just a bit!) frustrating. It is my favorite region, but I played sapphire, ruby and emerald on pc emulator SO MANY TIMES, I knew most of the story and places, plus there was Platinum I recently bought that was waiting for me, so I played it mostly for the sake of finishing the game getting a masterball and cloning the heck out of it was a motivation too lol.
As for the new games. Platinum was amazing, I went super slow with it and had no problem finishing it. I also probably would mind replaying it, the game was amazing.
Soulsilver is awesome, but a bit slow. I am playing it slowly and taking my time to enjoy the game fully. Also I am familiar with both regions and Pokemon, but it doesnt take away from the game. The only problem is the Team Rocket puzzle. Obviously main reason for this is that its a remake of Crystal. I love the story, Team Rocket, and entire Johto and Kanto region, but this part where you battle TR grunts in Radio tower and then in the underground is plain, 100 grunts with rattatas and zubats are just annoying. Not boring, dull, or offpissing, just annoying.
Nevertheless, after this part I will certainly enjoy the rest of the game. Kanto and Johto with the new graphics are amazing.
Everything later is yet to be seen. I have Pokemon White ready and its going to be the first time I am playing a Gen 5 main series game :D
As for the other games.
I could replay Pokemon Ranger. The game is quick and awesome and really fun.
Pokemon mystery dungeon....ugh. So Pokemon mystery dungeon, in my opinion, has by far the best story, but the idea of having to go there and grind all from the beginning is so repulsive. I could go through the story again EVERY SINGLE DAY, if it wasnt for that enormous amount of time you need just to grind your Pokemon and do all the trivial missions.
I will however get my hands on the PMDEO Sky and replay that. I really want all the extra stuff Darkness didn't have, plus Shaymin :D, I dont think it's be that of a hustle. :D