I'm not well versed with bassists (and responded bfore reading other responses), but the first person that comes to my mind is Les Claypool from Primus. His bass work just stands out when I listen to Primus.
Being this is an Opeth forum, I should also mention Martin Mendez. One thing about me listening to Opeth, though, is I feel he more blends in then stands out. That is not a negative at all, as obviously bass is an integral part of Opeth. But the drumming of Lopez on the albums (and Axe live), and Mikael stand out for me.