Let us start:
1. BG2
2. BG1
3. IceWindDale
4. Planescape

I had a simialr idea and asked Tal to have a poll on the front page of this site. Of course, being a great Sorcerer he had already beaten me to it. You can find the details of the earlier poll on this post:
http://www.sorcerers.net/ubb/Forum2/HTML/001760.html

(You'll need to scroll down quite a way)
Although, strictly speaking, Tal's poll was 'which one have you played the most' which isn't the same as which one do you think is best or enjoy the most.

For more poll results have a look here:
http://www.sorcerers.net/poll_arc2.htm