The Bach Cello Suites are deservedly famous, but if you are looking to branch out to other solo cello music I recommend listening to Zoltan Kodály's Sonata for Unaccompanied Cello. After the Bach Cello Suites it is probably the most important piece in the solo cello repertoire. One of the unusual features of the piece is that it calls for the bottom two strings to be tuned down half a step which gives the cello a darker timbre.
https://www.youtube.com/watch?v=phygv_Et9sQ