for being the greatest warriors and hunters in the universe, they sure lose alot.
Arnold beat one
the main guy in the 2nd movie beat one
they lost in AVP and I haven't seen any of the other movies 

That's like a handful of Predators out of an entire planet's (or more) worth.

Not really Arnold and his team got pwned and they were an ELITE unit Arnold just got lucky at the end. As for danny glover the Predator in that movie was younger and was learning to hunt for the first time not to mention he owned another special forces team. Again Danny got lucky with the razor disk at the end. But if not for that he would of gotten destroyed by the other Predators at the end. And Avp never happened that was a joke.

he may have lost the avp movie but he killed like a thousand aliens before he died.