1. Favorite experience level is a range. I most enjoy levels 1-6 both as a player and a DM.
2. I like the lower level stuff as a player mostly because of the sense of danger around every corner. When your PC walks into a situation, the outcome is almost always in doubt. I enjoy putting together a character at level 1 and each new level gained then feels meaningful because the character is slowly becoming more powerful and is beginning to be able to survive the rigors of adventuring more easily.
From a DM perspective, I like running levels 1-6 because challenging a party of adventurers is much easier at those levels than it is at higher levels. Often I feel with higher level play that it is a crapshoot as to whether something thrown at a party will be challenging, overpowering or too weak.
3. As a player my favorite modules have probably been an old Mayfair Roleaids module called Shipwrecker, although I have to admit that we really went off-course in that adventure as a group but to the DMs credit, he managed to adjust to our antics. I also enjoyed I6 Ravenloft quite a bit.
As a DM, I really enjoyed running Red Hand of Doom. That module was nicely laid out, allowed the PCs a great deal of latitude in how they went about things and was just an easy module to run. For an older module that I really loved, look no farther than U1 The Sinister Secret of Saltmarsh. I thought it was a great introductory module with a couple of cool adventure sites (haunted house and ship) and I loved the way the town of Saltmarsh was presented. It gave basic facts about the town and left it for the DM to develop as he or she saw fit.