I reccomend BTCGuild! The best for me !
On the contrary, I reccomend you all not to use BTCGuild for the security of bitcoin network.
Have a look at how hashing power is distribuited:
http://static.blockchain.info/pools.png?format=png This makes possible a 51% attack just targeting the three biggest pools.
Someone can own BTC network attacking only three servers!
This is not how things was ment to be...