for me, it's a mix between ps2 and ps3, but i voted ps3 since i'm currently still buying games for it, whereas i stopped purchasing games for ps2.
i occasionally buy a ps2 game here and there if i find one i want/like, but i primarily buy on ps3.
i would vote pc, but i don't spend much time on my pc, nor do i keep it up to date, so it really can't play many games.
as for DS, i don't buy games, i got a r4, which allows me to download games and play it from there.