There are many in all of fiction above them, sure. But they're still in the top 90 percentiles, I would think. Their production rates alone (building planets which build robots which build planets, the kind of material required for that) are insanely impressive. Spending a lot of time in /r/whowouldwin definitely shows they're high up there. Any society with time manipulation no doubt beats them (as mentioned, Xeelee, downstreamers, timelords, Q, etc), and a bunch without it (War in Heaven Necrons for one), but just because there's a lot of fiction more powerful doesn't mean they're not very, very high up there in fiction. They'd probably stomp the Imperium of Man, and that's from Warhammer with its insaneballs fuck-physics attitude. Fiction is just so broad.
Great sub, by the way:
http://www.reddit.com/r/whowouldwin/search?q=forerunners&restrict_sr=on