Scarily I realise we have done all of those routes!
Loughrigg Fell: this is great to just ride from the door if you are in Ambleside. You can ride over to Elterwater etc and just join up some really fun routes, as Simon said above, the VG route is a good version of it.
Grizdale-Parkamoor: really good in bad weather - Lakes rock is always slippy in the wet, but this route is pretty mud free.
The Garburn Pass - good route as it is out of the way from the main walking routes, so a good option if the weather is good. I cant tell from the satellite plot of it which way it is going, but some of the fells SW of Garburn can get quite muddy.
Walna Scar: really fun (but I haven't done it since its been flattened). Dunnerdale Fells has some good singletrack on it. You can avoid climbing all the way from Seathwait if you take the bridleway along the wall from Stephenson's Ground but it can be boggy and then you miss out some of the other good stuff, but it is a good option is people are finding it tough.
Staveley - Nan Bield: I've only done it from the otherside however, it will probably be a long push and a tough drop off the top. I think the bottom at the Stavely end was supposed to have been eroded out a couple of years ago but I don't know if it has been mended. As said above you could link this and Garburn together. Plus anything from Staveley has great food and beer at the end.
I would really strongly recommend the VG guide.
Another good day out is the Skiddaw Loop and can be a bit more weatherproof than some other rides and doesn't have quite the height so you can avoid the worst of the low cloud.
My 3 would be:
Low: Loughrigg amble, Grizedale
High: Garburn, Skiddaw fig of 9 or even riding down sticks pass from Glenridding (see Benji's ride write up in international mountain bike mag)