I am a huge O'Sullivan fan - been following him for a decade or more - but for those who don't remember, his deity status didn't follow him from the 6x12 to the 4.5x9 when he came over here to play pool. He didn't even win half of his matches on the IPT. He is also not, to my knowledge at least, a 3 cushion champion.
There are a handful of guys who have been stone champions at two disciplines (Snooker and Pool for Alex, 3c and pool for Worst, etc.), or, in the case of Efren, have proven that they are so skilled that they can win championships in games they have never played (Efren won a straight pool title in 1995 - his first competitive effort at the game, AFAIK), and as much as I would like to see him on the list, I don't see how Ronnie can get the nod over any of those guys.
Edit: Lindrum hit his stride in the 20's and 30's, which is outside the scope of the OP's question.
If, by the title of this thread, we're talking all-around *cueist*, as much as I adore Ronnie O'Sullivan and his natural ability on the snooker table, like it was said, his ability didn't necessarily translate to another cueing sport (i.e. pool).
The incredibly great Willie Mosconi was a 14.1 specialist, and actually expressed hatred towards other games on the same equipment, nevermind showing an interest towards the other cueing sports.
So I would have to go with the following two:
Harold Worst (held both pool and 3-cushion championships simultaneously, and who knows what transcendental heights this master would've achieved if brain cancer didn't take him from us at so young an age.)
Efren Reyes (obviously, no matter if it's pool, straight-rail, 3-cushion, 14.1, One Pocket, etc. -- the man can play *anything* to a high level).
Again, this is in the spirit of the title of the thread, which separates the person and the cue from "pool" in general.