You ask that question to 10 different people I am sure you will get 10 different answers.
For some people the absolute best t-shirt would be thin and half-see through, while others would be looking for heavier weight opaque garments.
For some markets tight fit is a good match, for others - relaxed or baggy shirt is what's required.
I have some clients that like higher, tighter necks and others that want deep plunging V-necks
There are people that wouldn't care for anything, but cotton and those who find blends are more comfortable to wear, or if it's for hotter climate and active wear - they would want performance moisture-wicking tees...
It's all very subjective and largely depends on who are you catering for, what type of the design will go on the shirt, what is fashionable with your target market etc.