One of the most popular is the triple waterfalls at Aysgarth Falls. They're quite easy to get to and featured in the Robin Hood Prince of Thieves film. Janet's Foss by Malham is another well-known spot, also popular for wild swimming.
Cautley Spout tumbles down the Howgills above Sedbergh. It's close to the Pub with No Beer...Hardraw Force is also close to a pub - you access England's highest single drop waterfall via the Green Dragon pub near Hawes! Nearby is Cotter Force which has a good accessible footpath.
The Ingleton Waterfalls Trail takes you close to the edge of two rivers, with spectacular views of several waterfalls (fee payable). Catrigg Force is more hidden - it's in a secluded spot within a wooded gorge a short distance from Stainforth near Settle. Nearby is Stainforth Force where salmon gather each Autumn and leap the falls on their way upstream to spawn.
Linton Falls on the River Wharfe form part of a very enjoyable walk from Grassington to the pretty village of Linton. West Burton falls are just behind the village green. The waterfall tumbles down into a large plunge pool and is accessible from different angles, making for some good images like the ones above by Paul Harris.