Can't beat Chilterns mud for horrendous gloopiness. Ride a singlespeed in winter (the first winter I rode geared I got through a lot of transmission components), generally have to replace very little now (beefy chain, s/s ring). Having said which, I can probably count on the fingers of one hand the number of times I've been faster than 3pmh off-road over the last 6 months…
Love it though, it's all biking and every condition is just another challenge. It can't all be well-drained trail centres in just-right conditions. Have happily kept going through this awful winter, really looking forward to the mud drying out!