In reply to Dark-Cloud:
If it has to be a circular route I'd look at doing the Lochaber four from the car park at the end of the Glen Nevis road. Up the Ben via Allt Coire Eoghainn and Carn Dearg. Then CMD Arete to Carn Mor Dearg. Down CMD east ridge to the col under the Aonachs. Nasty steep climb (but signs of footfall make it pretty obvious) onto the south end of Aonach Mor. Quick nip across to Mor summit, then backtrack and down to col under Aonach Beag. Go over Aonach Beag then SSE onto Stob Coire Bhealaich ridge. Descend to col under Sgurr a' Bhuic (there's a clear path), then down beside Allt Coire nan Laogh into Glen Nevis at Steall. Easy stroll back to car park.
Nb. for a lot of people that is a monster of a single day out, so how long to allow for it depends what you mean by 'fast'
The Cairngorm 4000-ers in a one-er is a much longer route than the Lochaber 4s, but also a lot easier on the ground. Many people would class it as a 2-day walk:
Ski centre - Cairn Gorm - over plateau to Ben Macdui - down to col between Macdui and Carn a' Mhaim - on down into the Lairig Ghru - Corrour bothy (handy mid-point for an overnighter if you are spreading the walk over 2 days: bring a tent if you're there at the weekend - gets busy)... Next day Cairn Toul and Briaraich, then descend B via the Sron na Lairige ridge down into the northern entrance to the Lairig Ghru. Climb out the far side and go through Chalamain Gap, thence back to ski centre