Line 6 DL4, if you don't need a ton of time for your loops (i.e.- 14 seconds), it is easy to use and not too expensive. Also, the other delay models are handy to have too. It's just a really fun and useful pedal!
I have a Boss RC20 I use to record a loop and play leads, triads, arpeggios, etc. over it. I use its most basic functions only, but it has plenty of recording time (for me) and is easy to use (required).