MyPoints as quickest to reach cashout level. Like others, I always cash out at 1350 points for a $10 webcertificate. If you have lots of areas checked as interests, you'll get a fair number of Bonus e-mails. You get 5 or more points just for clicking the button in the e-mail.
#2 for me would be Blink. Though I haven't cashed out on this one yet (it takes 53,000 points to get a $5 webcertificate), it's easy and fast on a daily basis.
#3 is MileSource. You can get a $25 "Supercertificate" for 3500 points. This one changed recently and it now takes longer to accumulate points, but it isn't hard if you develop a "method." It also helps that Mal and others post the Trivia answers early in the morning (Mal, how early DO you rise in the mornings, anyway?)
It also helps get miles at Milesource if you do the Pick 7 and the Milehunter. With Milehunter, after every five unsuccessful clicks you are asked a trivia question. Open another browser window and do a Search to find the answers. The whole thing takes a few minutes, but if you have time (maybe five minutes +/-) it's worth 2 more points.