Arran Bouldering, a set on Flickr.
The biggest, meanest, longest bouldering circuit? I'd say Arran has a good chance of winning this one. Hop the 7am ferry, get a bus to North Glen Sannox and walk up into Coire nan Ceum, do about 20 of the best problems there (mostly slabs but a few choice roofs up to 6b), then nip up the Witch's Step to the Caisteal Abhail ridge. This has some fine bouldering all the way to the summit on some isolated blocs and on small tors beyond.
Drop down along the big arced ridge to the Cir Mhor summit and some architecturally challenging domino blocs with body-munching properties (and the Rosetta Stone, if you can find it). The silvery slabs on the south west flank of this hill have some compact rock and balancy problems.
At this point you might want to stop for some lunch, you'll be about 40 to 50 problems in. Got enough water? I couldn't find those springs marked on the 1:25,000 map, though I was convinced I could hear bubbling water under the stones.
Now drop down to Fionn Coire under the Rosa Pinnacle. Lots of fine easy bouldering on the howff boulders under the crag, then nip across the stream to the A Chir boulders, a fine compact collection of technical problems (and some big projects). About 30 or so problems up to 5c/6a.
By now, your feet should be raw meat, if it's a hot July day, and you'll be drinking water from the streams at every opportunity. Cramp-thighed, stomp down Glen Rosa to the Daingean boulders by the bivi stone at the path (Cuckoo pockets etc). About 10 problems here up to 7a, but you'll have no skin for that level.
A final haul down to the lower Rosa boulders (about 15 reasonable problems) and a well-deserved dip in the plunge pools of the Rosa burn, then see if you can make the 4.40 ferry.
To extend it properly into a form of alpine torture, walk over Goatfell's south ridge to drop down to Corrie and finish on this classic circuit and catch the later ferry (over 200 problems in total, about 15 miles walking). I made it to plunge pool in Glen Rosa and was done in . . . a blistering circuit, indeed.