Interesting question!
If I can only pick one then it has to be Sleeps With Angels by Neil Young & Crazy Horse. That's the ultimate up all night chilling with some bottles on somebody's porch album.
But I've had different summer albums at different points in my life. Led Zeppelin III, LA Woman by The Doors, Not A Pretty Girl by Ani Difranco, Wildflowers by Tom Petty. I guess I tend to go more for a relaxed feel over a hard party atmosphere.