Tough. As in able to handle the recoil. I haven't had much luck with scopes personally, maybe it's just my dumb luck, but I have had problems with a Bushnell that won't hold zero and a Nikon that the focus changes after a few shots. A Barska Tactical is the only one that seemed reliable on my gun, but after having it fog up when I needed it I have fallen back on good 'ol reliable rock solid A2 iron sights. No muss, no fuss, no battery, they are there when you need 'em rain or shine.
But if you're a scope kinda guy, or gal, don't forget one biggy to consider, eye relief.