I think it depends on the desired output. But I would have to say Illustrator for vector artwork and Photoshop for raster art and/or a combination of the 2. Talk to your t-shirt printer before you start so you get the exact specs required for best results.
Nancy is correct- check with your perspective printer before you commit to an application. Many have preferred programs and will charge you if they need to convert an image to a program that is their choice.
A very good point. Some professional printers are fussy about which software and version they want you to use. My print guy doesn't have Creative Cloud so I make accommodations to fit the versions of software he uses.