Ty Cobb. Closest modern player is Derek Jeter - and we're talking a difference of 800 hits, 800 - that is like 3 really really good seasons worth of hits.

So, whether you think comparatively he is the greatest hitter the game has ever seen or not, nobody can dispute he technically is the greatest hitter ever because he has the most hits in history.

Tony Gwynn - 1100+ hits behind Rose
Willie Mays - 1000+ hits behind Rose

Most no-doubt, all-time great hall of famers are in the 3000-3500 hits range. Rose is at 4256, just crazy.