Conti Garorskins in 25 with an old 23mm one with the bead cut off inside the rear tyre. Prevents punctures even when I ride it right down to the canvas, then I move front to back and put a new one on the front. I get huuuuuuge mileage that way. Grip is poor and the ride is harsh and slow, but the bike's a total dog anyway (tatty old fixed with mudguards extended with chopped up milk bottles) so kind of goes with it. Also makes me love my tubs when summer comes around. I put in a LOT of miles in in the winter, so the fewer times I have to get my hands cold fixing a flat the better. Happened once all of last winter, so I'm happy with this setup.