As far as their overall character writing goes, Muten Roshi by far since he had a lot more of a proactive role in the plot and some actual dramatic moments to go along with his comedic nature such as the 21st TB finale and his death against Daimao. If we were to include members amongst the Dragon Team that played a mentor role, then Saiyan Arc Piccolo would take that by far with his mentorship of Gohan due to his overall character development being one of the few cases where Toriyama actually bothered to use subtlety in their change.
In terms of how they work purely as a mentor/support figure, then probably Kaio due to him staying somewhat relevant after his mentor role in the plot was done unlike post-Daimao Arc Roshi, Karin and Popo/Kami, such as helping Galu find New Namek and the Genki-Dama being far more important in the long run than anything learnt from the other main mentors.
