A thousand different possible answers, but if you want an easy way in, start with Miles Davis' KIND OF BLUE and IN A SILENT WAY, then Coltrane's CRESCENT (A LOVE SUPREME is not a starter LP), and for something different try Eric Dolphy's OUT TO LUNCH.
I will say that, though it is extremely uncharacteristic (both as jazz and as a Miles LP), IN A SILENT WAY has been my 'gateway drug' of choice for tons of people. Nothing else sounds quite like it, and it's basically impossible not to like.
BITCHES BREW is a trap of sorts -- hugely influential, yes, and a great record, but also deeply weird and disconcerting if your only prior reference point is rock or even mere trad jazz/bebop. Also, it's flawed: the title track is a bore and "Sanctuary" is...not good.
A lot of people are citing Mingus's MINGUS AH UM in my mentions, and I'll happily endorse: a great jazz record, and not difficult for tyros to absorb. However, Mingus's BLACK SAINT AND THE SINNER LADY is arguably the most overrated jazz record in history.
