For me, it’s gotta be a tie between my Oompa Loompa shirt and my Clockwork Orange shirt. The Loompas get more comments, but once I was out in my Clockwork Orange shirt and this guy showed me his nice, full-color Clockwork Orange tattoo. (It was on his upper arm, ok? He didn’t have to disrobe for it.)
I’ve been considering getting some of the shirts from the comic referenced in my 'sig (and by gum, I’ll be wearing it to any and all 'Dopes I go to.)
Microsoft Bob
D
I got the most comments about a concert shirt from Nik Turner’s 1994 “Space Ritual 2001” tour. Hawkwind fans seem to be pretty rabid–every so often as I was walking down the sidewalk, I’d hear a scream of “Hawkwind!!!” or “Wooo!!! Nik Turnerrrr!!” from a passing car.
I also tend to get comments about a Residents giant-eyeball shirt.
It’s a long-sleeve t-shirt with a picture of a guy drinking a beer. It says “Reward Your Friends, Punish Your Enemies.”
I got it off of CafePress.com
Happy
Mine is a long sleeved tee as well… it’s got a rainbow with ‘evil’ in calligraphy beneath it.
Everyone claims it says ebil though, so it is now known as my ebil shirt.
~ebil Flutterby
My BotCon Japan T-shirts, simply for the excellent gritty art.
Unfortunately, I look lousy in black, so I tend to wear them only on special geeky occassions.
I had one that said “I am a pervert” in Japanese, but it never fit very well. [from www.jlist.com, which is most assuredly NSFW].
Now, I guess the Xavier’s School for Gifted Youngsters tee.
I also have a Crystal Method T that says “got meth?” on the front, but I’m usually too embarrassed to wear it in public.
My personal favourite is a custom-made t-shirt from school, known as the Ultimate T-Shirt. A few friends and I basically drew a cube in the middle of a page, then took turns adding attachments to this until the page was full, thus creating the ultimate thing. We put that picture on the back - on the front, there is a picture of Newgent, the Ultimate Sea Iguana. I won’t go into that here, but suffice to say that he rocks.
~ Isaac
My Bungie.org T-Shirt, followed by my “Billy and the Boingers World Tour” shirt.
The latter, alas, hasn’t fit for many years. 
“665
Neighbor of the Beast”
I swear it was an accident, I once wore this shirt into the local Christian bookstore. I thought I was getting strange looks because I’d never been there before. When I was putting on my seatbelt upon leaving I noticed what I was wearing. It’s funny, but I really hope no one was offended.
~S
My ThinkGeek T-shrt:
“There are only 10 types of people in the world; those who understand binary and those who don’t.”
This is one of my favorites. I have to look through my other 150 t-shirts to try to find a better one.
My simpsons t-shirt, with Groundskeeper Willie ripping open his shirt on the front and the words “Grease me up, woman!” on the back.
My shirt has a buncha sperm swimming around and egg and says “Future People”. The pic is kind of dull, really, so most people don’t look at it. Those who do frequently give me some unusual looks in response.
I have one with “It’s all Greek to me” written in actual Greek. I have a lot of fun wearing it on university campi…
My favourite “cool” shirt is my fornicating fruit & vegetable one. It’s white with a bunch of pictures of fruits and vegetables engaging in sexual congress all over the damned place. I love it! I’ve had it probably close to 10 years by now.
I have a black T-shirt with the Phantom mask, and Phantom of the Opera/Majestic Theatre/January 1988 on the front, and “I don’t care how they did it in London” on the back. They were made up by the crew of the Broadway production, who got sick of hearing the principals who had done the show in London tell them how they did it in London. The show is very threatre-specific, which is why it hasn’t been done more.
I also have a gorgeous blue sweatshirt adverting the London “Blondel” (thanks, Arnold). Someone once offered me $50 and a new sweatshirt of my choice for it.
My best has a quotation on it:
“When I get a little money, I spend it on books, and if there’s any left over, I spend it on food and clothing” – Desiderius Erasmus
Given to me my my mother, swho clearly knows me all too well.
The one I’d like to be my coolest is one I made myself. It has the construction for the “witch of Agnesi” (a mathematical curve that’s basically a Lorentzian). Underneath it is a horizontal line, with “The Witch of Agnesi Project” written elow it. The style and calligraphy copy the poster for “The Blair Witch Project”, and the construction of the curve has been made to look as much as possible like the rune they use for the symbol in the movie. I made it myself, since I couldn’t find anything like it. I hoped it would get an appreciative nod, at least. But I’ve worn it to SF cons and MIT without a glimmer of recognition.
(Sob!)
Mine’s black, with a white, red-eyed Cthulhu on it. He’s saying, “Sell your soul for a cookie?”.
It’s great.
“i Kissed The Lead Singer”