This is like that question about naming your top 5 fave songs - it makes my head explode trying to choose. So I'll pick the last good one I did a couple of weeks back to stop my brain frying.
I reckon the downhill into Boot from Burnmoor Tarn was blimmin good fun - 2 of us did it v early morning, not a walker in sight. One of those does life get any better moments. I was grinning my face off at the end of that one.
The long bridleway into Seathwaite (Styhead Gyll?) is pretty good too.
Never done Nan Bield and that looks ace - we had to can our last attempt at that cos someone fell off big style beforehand forcing a re-plan etc. But's it's defo on the list of ones I think I must try.
I love riding in the lakes per se - it's just brilliant for big days out pretty much all year round.