For your short day then Warnscale by Buttermere is excellent fun- short but very exciting. Start at the slate mine, up the brutal fireroad, down to Warnscale Bottom then back up the road. You can work this into the Borrowdale Bash but why spoil an exciting trail with a load of boring fluff?
My favourite full day ride probably depends on your ability and tolerance of cheeky riding but up the Skiddaw bridleway then down Longside Edge to Ravenstone, back up through the woods to Carsleddam, down to Millbeck then back up the bottom of the Skiddaw bridleway is excellent. Much steepocity, looseness, big rocks and naughty footpaths but very, very good.
With regards to geetee’s ride avoid adding Gatesgarth, which is just a landrover track and a waste of good descending time, into the mix- you can do this by turning left and going over Harter Fell before you descend to Haweswater.
If you’re good on a bike up and over Nan Beild and back is excellent. Or over Gatesgarth, up High Street, down Mardale Ill Bell, down Nan Beild to the north then back up Nan Beild and descend to Kentmere back to where you started.
Helvellyn is also a stonker.