Just ordered a 3-pack of grey v-neck 100% cotton tees. I’m set! 
I change into workout clothes when I exercise because I really don’t want my regular clothes to end up smelling, even after they have been washed. Usually it is sweat wicking clothes whether at the gym or at home. At the gym, I do not want to look out of place, so that is a little of a consideration and if wore just a regular shirt and pants I would stand out. I don’t go for anything form fitting or whatever though. I do try to color coordinate a little bit though.
//i\\
As mentioned in a previous thread, I do like to wear eye catching (but practical) clothes to the gym.
Because it almost feels like a uniform, everyone dressed in grey or black under armour.
So I wear things like a math-themed vest, or Jamaican flag, and of course Christmas t-shirts for Christmas etc.