It really depends what kind of mood I'm in. I probably put the most time on Oktoberfest, but that is because I can get a really long game out of it. Houdini and Oktoberfest are similar with tons of modes and with lots of different strategies. I love them both. Hotwheels is noticeably different in that at its surface it plays fun and fast, but it is deceptively deep. I love how dangerous the rpm targets are and getting to 5th gear is no joke. Hotwheels story is silly and and fun for everyone who plays it. If I want a quick game I usually play Hotwheels.
I am still figuring out Houdini. I have a unique perspective as I bought these pins in reverse order. My first was Hotwheels, then Oktoberfest, and Houdini last. So I haven't spent enough time with it to give an honest opinion. All I can say is a few shots are very challenging and very rewarding, and completing modes are somewhat difficult for me. I can't get enough of locking a ball in the trunk though.
Right now I'd have to say I enjoy Oktoberfest the most because of its challenging ramp shots, it's funny and quirky call outs, and it feels very rewarding to play.