Honestly, I'm normally not that fussed. I have a Priority Pass, and have tolerated some poor lounges. As long as I get a reasonably quiet space, a comfortable seat and the beer/wine selection is palatable, I'm happy.

That said, I do like the ever so busy Galleries South in T5. It's huge, and if you walk far enough North, it gets quiet enough to enjoy (after I've had my curry).