You chose....The Constant?? Well colour me shocked.....

This is a tough one for me. I guess I would have to pick 'The Constant' as well - from these choices anyway. I thought the two-hour series premiere was one of the best pilots ever made at that time. 'Through the Looking Glass' was the best season finale for the series. The most shocking episode that really caught me off guard was the Season Two opener with Desmond in the hatch - that was such a fun surprise because you were trying to guess who's flashback it was.
There are quite a few episodes though that I liked better than a lot of these listed, but of the ones listed, I'll go with The Constant.