Think pokerstars is the most attractive for a beginner poker player. Run most games by far. Here you can play for play money, but there are also freerolls. If you do want to deposit money, pay attention to the first deposit bonuses that are offered.
My favorites are GGPoker because it offers many games that are not found in other rooms such as Flip & Go where just by winning a hand you can enter prizes and bodog for the freeroll tournaments they offer
Americas Card Room for USA players since it appears to have the most traffic considering what software is available to players in the US at the moment.
I was barred from Global Poker and 5dimes.eu. because they stopped accepting US players. I don't play on any US sites simply because I happen to live in an ultra liberal state. My online play is limited to sportsbetting.ag, where I multi-table a lot of the time in S&G tournies. This is probably not the answer you're looking for but it's the best one I've got.