My favorite sensor, hands down, is the lowly limit switch. You can do just about anything with these things. That and they are cheap, tough, little buggers.
My second favorite is a broken beam sensor. I don't know the make or model of the ones that we use. They are easy to install as well as easy to program. They also glow red in the dark, which you know, is kinda important
You have to be careful with the broken beam sensors, however. If you order some, make sure they use LEDs for the light rather than a laser. Also, you get improved reliability if they are in an environment where they are not exposed to too much direct light.