What's the difference between normal cotton, baby cotton and mercerized cotton? Which one is best for a shirt?

Mercerized cotton is treated in a way that gives it a different sheen and texture than regular cotton.

Baby cotton, as I understand it, is just a finer weight than worsted cotton. :shrug:

Mercerized cotton is also sturdier and a stronger fiber.

I'd used mercerized for a shirt since it has a nice finish and drape to it. But I've used un-mercerized too, it just has a much more casual look.