Short answer: Not much, mostly because I don't set a limit of 10.
This is my current top 18 in alphabetical order, exceptions excluded.
Long answer: I keep an Excel document* which numerically categorizes how I much I like each villager's appearance, personality, house (exterior, interior, and music), and how much 'work' it would take to make their home more appealing. Their average score automatically ranks them, and I can sort the list to show the highest-ranked villagers. I do make exceptions of convenience though. Lucky's house is awful, and I still don't like his wallpaper/carpet after replacing most of his furniture, but him moving in (without camping) on a perfectly-located plot was too good to reset. With him and Shep, I now had to bump Butch since I didn't want 3 dogs. The same thing happened with Freya when I already had Skye.
I also wrote a small program that compared two villagers side-by-side, then ranked them according to whom I chose most often. It was similar to the
Pokemon Comparer. However, Excel is easier to change and keep track of. I've used this document since the GameCube version, adding columns as necessary.
All that said, the top 15-20 ranks change frequently but the top-of-the-top candidates have been pretty consistent since I started playing in March. I also try to keep them diverse in terms of species and personality.
... I think I'm some kind of monster. Some horrible, analytic, systematic monster. Like an accountant and a bureaucrat filed the necessary paperwork and procreated.
*(seriously)
