In terms of Best Online games sites, it depends on what kind of content are you looking out for. Is it Casual Flash games that you are looking out for? or is browser based RPG's or are you looking for specialized content?
In my opinion below are some of the best sites in terms of Casual Online Games, in no particular order.
Some really nice addictive flash games in the genres of Romance, Car, Cricket, etc. They also have a different type of content called Naughty/Funny games which are not available anywhere else on the web.
Some of the best collection of games suitable for adults and kids. Quick, casual and snacky games.
Miniclip does not need any introduction or explanation in terms of content. They are king and the first movers in the online flash games business.
-Girlsgogames.com, agame.com, etc. (Spil Group)
The masters of SEO, these sites have some of the best games categorized into different genres according to their sites. A huge collection of sites around the globe featuring games specifically for that region.
Some of the best collection of boys & puzzle games you will ever see.
-Kongregate.com (recently acquired by GameStop)
The best platform to see new games by talented developers because of their developer awards program.
Some of the best Parking, Shooting & Cricket games you can find on the internet.
The kings of online browser based MMORPG's
I think you'll be mesmerized and spend enough time playing games on these portals so I wont mention anymore.
I head the Indian Business Alliances Segment & also lead the analytics part for Games2Win a Top20 Online Casual Games company so maybe the answer could look biased.