Hi, Debonair Damsel.
I love original Doctor Who (Hartnell through C Baker. McCoy was ok, but some of the episodes were a little weak). Impossible to pick a favorite episode/story arc, although have probably seen Fang Rock the most number of times.
I tried the reboot series for a little while, but it doesn't have any vibe of, or stay true to, the old series. Two thumbs down.
Original Star Trek and Next Generation are also favorites. Have seen NG episodes so many times, I try to see how many seconds into an show I have to go to identify which episode it is. A couple (of many) favorites are The Naked Now (based on Naked Time from original series) and All Good Things.
Voyager was ok, didn't get too much into Deep Space Nine... it was pretty funny that two actors from Benson were on the show, and Enterprise was flat out awful.
Not sure if it counts as sci fi, but one of my all time favorite shows is Mystery Science Theater. Used to have a weekly house party when STNG and MST were on Friday nights. Of course, it's Joel over Mike; who didn't tear up for the Mitchell episode? The show also lost a lot when TV's Frank left, followed by Dr Forrester, and I started watching it a lot less. Again, hard to pick a favorite episode because so many are hilarious. Manos is obviously amazing. Side Hackers is funny, as is Pod People. Anything with Gamera I love, too; he's filled with turtle meat :-D
-MRB