In terms of a flawless back-catalog, there isn't many....DMZ is close to perfect in my mind, but then # 012 & 013 came along. So the only one I can actually think of would be Burial Mix/Rhythm & Sound where I could draw any release from the shelf and put on any track and not finding one weak one (then again, it's a 'single' artist label...).
edit: thinking again, this sounds arse. obviously there's tons of great other labels. But thinking about it while writing I honestly couldn't think of anything close to those 2 and being still impressed with every release (and again, having only 10 odd releases of a single artist makes this simpler than releasing 100s or different artists' releases...). No doubt the previously mentioned as Warp, Rephlex, Ninja, Reinforced, UR, Metalheadz, Planet Mu , Hyperdub, Tempa are all great labels, but all also have had stretches of good/bad releases or even full on shift of scope (Metalheadz) or even loss of scope/vision (No U-Turn) which didn't go down well with me at all ... but to contribute and add a few more that haven't been named yet - as far as multi-elektronic-(now)-pop-sound a la warp/ninja goes - Leaf, Lo, Plug Research, Kitty Yo, Nonplace all had pretty good runs as well. As for some genre specialisations - Sähkö, Raster Noton, Staalplaat - Whitehouse, Rugged Vinyl/Intense, Science (for the Source Direct & Photek releases) - soundkit/soundhack/mmm, fxhe, workshop are all mighty fine.
c.