HeartGold/SoulSilver, I don't know I guess the idea of your Pokemon following you like in Yellow was fun. <br />
plus, it's got an old school feel.
Pokemon Snap. The handheld ones are good, but after a while became painful (gave up catching and breeding them all after gen 5). For pure fun Snap was probably the best, followed by Rumble.
None of them, just a simulator; pokemonshowdown.com