Besides the freerolls 888 Poker also have many tournaments, where the fields are not overly large like on PokerStars, which makes it easier to reach a final table. Tournament rake is also lower, and I think, the competition is softer overall.
I have no recent personal experience with the other two sites, but judging from this forum more people complain about various issues on ACR, and to me the software is not particularly appealing. PartyPoker is one of the oldest players in the business and might be a good choise, if you prefer playing without a HUD.