I have a few different 308 gas guns at the moment and it really all depends on what you are going for
I would rank mine as follows.
LMT MWS, its modular, it can go from a steel cased blaster to 6.5cm in about 60 seconds, its a little heavier than its pears even in its lightest configuration, but is fine as long as keep the rifle somewhat slick.
Kac SR25 ACC, Just a very nice shooting and handling rifle, its light and has great practical accuracy, they have a great track record of downrange use. does it shoot twice as nice as some of its peers, I would say no. If funding no object KAC would be my first choice.
Scar 17, its light, its reliable, it folds.
The Scar really doesn't do anything for me and I am eventually going to sell it to a friend the next time I get back to the States.
I also like the POF Revolution and JP's 308, but have not had the chance to mess around with them. Barret and DD both have nice 16 inch gas guns as well.