Frontline but it needs to be applied every two weeks PLUS (as advised by our vet), a daily check...... Sounds time consuming and costly right but it sure does beat the $1200 vet bill we had last year for the treatment of our dog after he got a paralysis tick.
I have also used a tick collar that I got at a pet produce store and I think it was about $20. The collar can't get wet, and I can't remember how long it lasted (or the brand sorry), but I thought it was quite good as well.
At the end of the day though, nothing beats the daily check. Hope that helps.