I'd want it to be made out of something comfortable. I'd want it to be colourful. I'd want the slogan or picture to be not just funny but also clever, not just a funny catchphrase from a tv show that in 10 years time will mean nothing to anybody, but a clever joke that will survive the aging process. I'd want a large choice of styles, then even if you had the same joke on the tee shirt you would not neccesarily have the same tee shirt on. Personalization would be good, maybe a funny tee shirt with a wanted poster on it but with a blank part where you could use a tee shirt transfer to put someones face on it, maybe yours or a friends etc.
It would need to be made of a soft comfortable material that would wash time and again without losing shape or fading. it would have to be in colours that suit me and the funny slogan would have to be hilarious and of a type of humour that would not date, nor be offensive. i would want it to be laugh out loud funny and cheer everyone up. I used to wear a badge years ago that i thought said, Guinness makes you fat, and I was bemused because people used to laugh heartily and I used to think, its not that funny! One day i looked at it properly and it actually said Guinness makes you fart! So i suppose I would like to be sure what my teeshirt really said. And of course, the teeshirt must be flattering and make me look thinner!