Only 16X. Anything more looks like to much realism trying to fit into a patterered, unrealistic world. How often in nature do you see the same dirt patch over and over? If you make it more than 32 X 32 it looks bad. Painterly 16x is the bomb for me. Everything looks nice, blends well, and has vivid colors.
I've got a personal-use 16x16 texture pack made from tons of textures I ripped from the best TPs. In my opinion anything other than 16x16 is ugly as hell and should get the creator permanently banned from the universe.
32 x 32. I like the 16 x 16, and In fact I installed the 32 x 32 just because I was bored and wanted a change... Turns out not only it looks better overall, but actually INCREASES my FPS!!... not by much, I say 5 FPS, but STILL...
I used Default for months. Then I found a nice pack called Shreeyamcraft (16x16) and used that one forever, it was simple but I loved it. Recently ive been using Jolicraft though I may make a painterly soon.