My favorite size is the 36, due to the capacity I like to have at my disposal for a work bag, plus the fact that it slouches so beautifully.
Been the 25 and the regular, I definitely prefer the regular (again due to capacity). The handles on the 25 also don't fold in well, and I find that I don't like the proportions of it as much as the other 2 sizes - it seems too thick for the height and width.
These are all personal preferences though. You will have to decide which size best suits your needs!