According to the syllabus put out by RCM, which has now partnered with Carnegie Hall to create "The Achievement Program," the easiest is WTC I C minor, listed as a grade 9 piece.
The Grade 10 Prelude and Fugues are listed as:
WTC I
D
Dm
E
Em
F
F#
F#m
Gm
Ab
G#m
Bb
B
WTC II
C
Cm
C#m
Dm
Eb
D#m
E
Em
Fm
G
A
Am
Bm
The rest are ARCT, which is the most difficult level.
Hope that helps!