Alot of the great one's have already been mentioned, honestly one of my favorite returns was when Austin made his return in 2001 to 'save' team WWF from the Alliance, the announcers totally sold that it was a legit suprise (atleast in kayfabe). I remember watching it and going absolutely nuts. I also love when he came back to be the GM of RAW in 2003?
Bischoff's WWE debut was pretty epic also.
A few of Taker's returns have been good and a few have been duds, but honestly I loved his 2004 return as the Deadman. I was starting to get into the internet wrestling sites around that time so I had heard rumors of it happening, but when it actually happened I have to admit I marked.
I also love Sting's TNA debut, and his last 'return' in March when he turned heel. Both were pretty cool if you ask me.