I've only played on three platforms: Full Tilt,
pokerstars, and 888poker.
For me, the best one was always the one where I won the most — and that was Full Tilt.
Unfortunately, it stopped existing after PokerStars took it over.
My second favorite is 888poker — I still make it to some final tables there, though not often.
But on PokerStars, I can't seem to win much, except for some Spin & Go's and small sit & go tournaments.