I suggest you start with Batman Chronicles, Vol. 1. It collects the first twelve Batman stories and introduces you to the original incarnation of the character.

From there, I don’t know what to suggest. Everyone raves about the Bronze Age stories written by Dennis O’Neal and drawn by Neal Adams – they’re the first comics to return the character to his dark roots after the Silver Age comics lightened him up – but I haven’t read the majority of them, so I can’t say from personal experience just how good they are. Also the current reprints of the stories have altered art, with redrawn panels and ugly digital colouring.