There are a lot of things you can use, but it really comes down to one really good accurate choice,and that is Trackmate.
I have two systems, a six lane IR Sensor on the oval ,and a two lane IR set up on the road course. neither have ever missed a lap, or double counted. I would recommend this system to anyone wanting to spend the money for a timing system.
Read this review Harry had done, it is a no BS, straight forward install, with great product support.
They are not cheap, but quality rarely ever is.
PitRow/TrackMate/TrackMate.htmBob