Paid:
I think Newtonica is really fun, and after hearing it's intended meaning I appreciate it even more.
Free:
Touch the Numbers; my friend and I got into pretty big competition with this game for a week or two. First heard about it through Gackt's competition with Masahiro Inoue haha. My record is 7.104 seconds. I challenge any of you to beat that.
I'm pretty rusty now though ... I usually get 8-9 seconds.
I wouldn't call either of these my favourite, but I enjoy them.