Toughest is probably Hardknott from Eskdale or Wrynose from Little Langdale. Worst descent is probably Wrynose to Little Langdale as its steep, straight, shite road surfaces, usually traffic.
If you've got a compact chainset then all the climbs are fairly straight forward (unless you're trying to race up them!) The steep bits dont last for too long so you can just do them all out of the saddle.
Fred Whitton covers most of the main passes and is a good route. If you're riding it with a group of friends then I'd guess it'd take you between 6 and 8 hours. You could add on some other bits to make it a full day.
Other steep hills off the top of my head are Strawberry Bank (as above) from either side (worse from Windermere side I reckon), Kirkby Moor, Climb from Hall Dunnerdale to Broughton Mills, Corney Fell, Birker Fell, Red Bank, The little pass at the End of the Langdale valley road.