decide which is more convenient
You'll need to be a bit more specific with your question.
A person just starting to play poker? Did you want to play in the smallest buyin games?
Honestly I'd recommend
partypoker. The software is good. THere's a decent number of games for micro stakes levels (there's also a decent number of super micro stakes & freeroll satellites too). They payout very fast on Withdrawals and never any messing around there (unlike say 888Poker).
Personally I'd suggest playing on All of them. My least favorite on your list is WPTglobal. (software is garbage, not enough players on the site). THey do pay out quickly though and there's lots of new players (rookies, crazies, etc.)
pokerstars has too many strong players.
888 also has very weak players at the lowest buyin levels but I'm skeptical about their payout speeds (You'll read about many folks who don't get paid right away)