Wow, thats not an easy one. Some people will go for the best technician, others for the best songwriter, others for the best entertainer.....
Since the question is "favorite" bass player, I'll go for the ones who influenced me the most.
1- Steve Harris (Iron Maiden): I begun to play bass in 1989 because of him, period. The speed, the intricate yet playable bassline, the sheer majestic power of Iron Maiden music made me a instant fan.
2- Geddy Lee (Rush): Made me discover progressive rock.
3- Roger Waters (Pink Foyd): Songwriter, pure genius.
4- Rex (Pantera): Never heard such a tight band. Flawless execution.
5- Cliff Burton (Metallica): Well, its Cliff, come on...
I also want to mention people like Cliff Williams (AC/DC) and Adam Clayton
(U2). While not being main attractions, they are absolutely essential in their respective bands. Knowing their role is essential for the magic to happen, as too much stuff would overshadow The Edge and Angus. We forget too often those silent type players.