Well, i just put some racing seats into the mach 1, and their really comfortable. If you are a bigger guy, id suggest cutting down the seat platform to make more clearence, as most aftermarket seats are taller.
Do a search on "what kind of seats do you have?" and you will find this thread that has many photos of different kinds of seats in mustangs. This is what helped me decide for my purchase. good luck, Todd P.
If I put the link in here it makes the window very, very large.
You should fill out your profile with info on your car - that would help us answer questions.
If you have a 65-68 and you want to retain the vintage look, Procar by Scat has a good seat. Look here for some info. Billgear (of the VMF) has these in his 66 and they look and feel great. I have the Sport Recliner from them (has since been replaced by the Rave 2000 - here's a picture of my seats).
Whichever way you go, you may end up wanting or needing to lower the seat platform to give more leg room. I'm not particularly tall (5' 11"), but I my legs used to hit the steering wheel (my seats are narrow and have very high bolstering, so I couldn't put my leg out to the side).