If you are looking for one-stop, do-it-all Android 5.1.1 custom ROM for your AT&T Galaxy S2 SGH-i777, definitely check out the latest SlimLP ROM.
Based on the latest Android 5.1.1, SlimLP ROM is the Lollipop sibling of its original Slim ROMs. And if you guys have been following GalaxyS2Root.com for the last few years, you probably remember that Slim ROMs started with the SGH-i777 (yes! This is where it ALL STARTED!).