I've been playing around with different ones. What I was using initially was 2 cups water and about a tbsp of baby wash and a tbsp of baby oil (I use Burts Bees baby wash and their baby apricot oil because I love the smell) and few drops of Tea Tree Oil. Then I was using Weleda Calendula baby wash with olive oil. Now I've switched to using coconut oil. I just dissolve it in hot water and as long as it's used in a wipe warmer, it's great. This is my very favorite oil to use and solution yet. I don't think I'll be changing it anymore. Basically, I always make my solution with a bit of soap of some kind, some sort of oil and a few drops of TTO keep it fresh, but since coconut oil has antibacterial, antimicrobial, antiviral AND antioxidant qualities, I've left the TTO out because I don't like smell of TTO and it's so strong that if you put one or two too many drops in by accident, it changes the whole scent of the solution. Yuck. I've also played around with witch hazel and aloe, but I wasn't crazy about those.
I know people who make their solution with ONLY coconut oil and leave out the soap because their babies have such sensitive skin.