Both Kallis and Sanga are hugely underrated - I think it's more a reflection of their relatively quiet personalities both on and off the pitch than any lack of talent. Each are very technically correct batsman who play traditional cricket strokes with little flair and flamboyance but much consistency and effectiveness.
Watched Kallis in the Big Bash the other night and he had sixty-odd and a strike rate in the very high hundreds but you'd have barely realised he was there... then he opens the bowling and still gets them through in the *mid-high 130's without breaking a sweat. At 39 years young.
*I do have some doubts as to the accuracy of some of the Aussie speed stats lol