Thick Stouts

Interestingly, water-ethanol mixtures have a higher viscosity than either water or ethanol do alone. A bit of googling shows the maximum viscosity occurs at a ratio of about 60-65% ethanol by volume.

I've had some high ABV beers that were thin and some low ABV ones that were almost chewy, and, as previous posts have mentioned, the perception of a beer being thick has to do with more than just viscosity.