it really depends on the nich of your site.. if the referral related to your site, you will get a higher ctr, and a higher chance of getting paid based on the action followed by he click..
I'm partial to the FireFox with Google Toolbar referral, and the GooglePack 2nd after that. Since they are both free they can be easy conversions if your visitors are interested in it, and if you can 'sell it' ..
Firefox and Google pack pay peanuts if you visitors are not from the USA, the usually pay you only 0.10$ for those conversions.
i stopped promoting adsense since their new policy ... I love referring people to bidvertiser ... a LOT better
I just started using the Game Poll Review referral which gives 6.00 - 7.50 when some one signs up and makes a poll. Something like that. -F0N3Y