Done:
Scarborough-Whitby - About 20 miles, the route is intact almost entirely. Very scenic. Steeply graded up to 1 in 39!
Waskerley Way (Crawleyside-Lydgetts Junction via Parkhead & Waskerley) - great route, with options to continue to MCE/NCL, DHM or CLS/SUN. I've done all except not yet done Annfield Plain to Sunderland.
Hartlepool-Seaham (inland route, via Hesleden/Haswell) - worth doing, but not great.
Manifold Way (Hulme End-Waterhouses) - Very scenic, but also very busy (especially when within 1 mile of a car park). Virtually no evidence that it was a railway line though.
Leek-Ryecroft Gate - Continuation of the Churnet Valley, can't go any further towards Macc unfortunately, although if you like wallowing in mud it looks possible to get a bit further! The Rudyard Lake narrow gauge railway is alongside part of the way. Reasonably scenic, and worth doing.
Biddulph Way (Stoke-on-Chav to Congleton) - the first few miles are to be avoided unless you like broken glass and chavs. But north of Biddulph it was quite nice. If I was to do it again I'd go from Kidsgrove to avoid the chavs and broken glass.
Middleton Way (Macclesfield-Rose Hill) - worth doing; though not very scenic as it's mostly tree-lined so you don't get a view, but very pleasant. Macc-Bollington is rubbish but the rest is good. MDL-RSH is very fast; I did that bit in 8 minutes!
Doing this Summer:
High Peak/Tissington Trails - 2 routes that join at Parsely Hay. Brilliant trails, I've done a small part of the Tissington Trail (Sparklow to Hartington) and it was great and I know (from photos and others) that the rest will be great too!
Ryehope-Stockton (inland route via Wingate) - only done the bit that's common with the above mentioned route from HPL-SEA. I am not expecting great scenery though, but it's worth doing once I'm sure.
Downs Link - 2 routes; the Steyning line and Cranleigh lines which both joined the Arun Valley Line at junctions near to Christ's Hospital station. Not done any of it yet but there looks to be plenty of interest along this route.
also see
http://yorkie.southtpe.co.uk/cycling/