In an event, under my wetsuit, I wear my Orca Tri Jersey and Tri Pant (shorts). Â When I finish the swim I just have to peel off the wetsuit, throw on bike shoes/helmet/gloves and I’m off.
The shorts are like bicycle shorts but with a much thinner chamois, so it dries within minutes of jumping on the bike, and there’s a pocket in the back for gels. Â The jersey is super breathable and has two massive pockets in back. Â Both are very comfortable and don’t chafe in the slightest.
In training, I use just the jersey with running shorts.  A cotton t-shirt is fine for up to 45 minutes, but past that there’s chafing, and chafing is lame.  Lycra and spandex are your friends.