Franck: Fugue from Prélude, Fugue et Variation
César Franck: Fugue from Prélude, Fugue et Variation Op. 18, mm 79-81 and 117-128
The Prélude and Variation are fairly straightforward in terms of fingering, but it took me ages to get a really legato fugue. Here's one bit: in bar 80, the RH G-F# is awkward whereas taking the G with the left hand is effortless. Alternatively you could play RH G, LH F#, but I prefer to end up with RH holding the F#.
Sharing the middle voice between LH and RH is the only way to make this fugue really legato.