It kinda depends upon what you're looking for in a site.
My favourites >
Fulltilt
for satellite mtt's, 45man SNG's, 27plyr.SNG, & 9-plyr SNG).
MTT's on Fulltilt... meh.. I don't really like the structure of the ones I can afford with my bankroll (ie. the biggest ones I can actually afford without satelliting are the $26donkaments and the structure in them sucks,.. it's a shovefest too early on imo). They do have some newer (cheap) SuperStack tourneys though... lots of weak players and lots of time to outplay them.
Pokerstars
MTT's - large field mtt.s (5,000 players is practiaclly the norm for the guar. tourneys in the daytime.) most have 3,000chips 15mins. blinds. Final table payouts for even the micro buyins are quite big (ie. a guy I know just recently took 2nd for over $2,000 in a $2.50 buyin... it was his first MTT on Stars).
SNG's - Micro buyin SNGs on Stars... I HATE 'EM. The play is soooooooo bad that the game is just bs imo. Yah yah yah.. we're supposed to like that (I find when there's too many of them on one table the game gets ridiculous and becomes ALOT more about luck then skill). I find I need to play at min. a $10 buyin and find them similiar to maybe a $5 on Fulltilt. To me the $20's (reg. speed) feel/seem easier to beat then the $3's (< I don't have patience for them).
Titan
Just started playing some tourneys on this site. So far.. so good. There's not alot of players on ipoker but the few tourneys I like on there have Great strucuture (ie. 6,000 chipstacks & 12min. blind levels... you're not even at 50/100 by 1st break & there's tons of room for play). These start at $33 buyin though... although there's decent $22 buyin MTTs too (3,000 chips, 10min. levels). They typically have 110-170 players in them & payout top20. The one thing I don't like about them is the payout structure kinda sucks (11th thru 20th all pays the same.. which means 'once the bubble pops & it's ITM.. it goes a bit insane until final table... because there's no benefit to getting 12th over 20th.
Another thing I don't like about ipoker - it's 10plyr. tables instead of 9.
The mtts $1-$10 I hear are decent to play in but they're all 1500chip starting stacks... or 'rebuy's. I don't play them very often personally but have heard they're ok.
SNG's... I don't particularly like them (10plyr tables again) and rarely play them at all.
Ring games "low limit" (ie. 2nl to 10nl) these tables are decent, you can find a half dozen or more to sit in on and there's always some really loose passive players & others who are way overvaluing their hands. I believe a micro limit player could easily build a roll on this site by playing micro buyin cash games (compared to Fulltilt for example).
The Rest
I rarely play on anything besides the above 3 sites these days (used to play on Doyles but these days I'd never recommend anyone deposit & play on there.. ugh). Carbon (Merge), you hear alot of micro players do really well on there and the guar. prizepool micro buyin mtts often have added value by not having enough players to meet the guar. I also hear that the play is generally softer & weaker than many of the others.
Brad Booth - > "Like a fight... it's not how you start, it's how you finish"