>>54800294HGSS > Platinum > Emerald > BW2
HGSS has the most content. Gameplay is pretty good. TMs are locked behind Voltorb Flip which sucks a bit. Nothing really wrong with HGSS.
I like some aspects of Platinum's gameplay more than HGSS. TMs are easy to grind for. Out of these 4 games I'd say Platinum has the best grinding due to the VS Seeker. I have to put it beneath HGSS due to saving lots of data though.
Emerald is a bit mediocre, I don't really like Hoenn much at all. Hoenn's battles are ok but I don't care for the characters or pokemon personally. The Battle Frontier is fantastic though and it's the only reason I would ever pick back up a Hoenn game, but if FRLG was designed like HGSS and they put the Battle Frontier in Kanto I honestly would not play through Hoenn again.
BW2 is just fine. I don't think its facilities are as good as the Frontier and I don't think the maingame is as strong as Plat's or HGSS's.