What route is best depends on your ability and preferences,the borrowdale bash offers plenty of challenges,Skiddaw is nice to tick off but not really a great riders route imho,The skiddaw figure of eight is a good route, Keswick railway track to Threlkeld,Blencathra centre,Glendaterra to Skiddaw House,Mosedale to Mungrisdale,Northern Mine tracks to Nether kellet,Whitewater dash to Skiddaw House Lonscale terrace to Latrigg car park, Spooneygreen to Keswick.
One of my all time favourite days out is:
Stbees to Enerdale,Black sail YHA,Blacksail pass,Wasdale head,keep right of beck to Styhead pass,Great climb to Angle tarn,Rossett Gill (VERY technical and steep) Langdales,Jenkin Cragg,Garburn pass,Stile end,A6 to hucks brow,Mosedale to Tebay. about 12 hours riding but one of the best days mountainbiking you will ever have. 😉