The Discworld series is not really a series in the traditional sense; the only thing all the books have in common is that they’re set on the same planet. Within the series are several sub-series dealing with different sets of characters, and several “one-off” books with characters that do not appear in any other books. It’s helpful, but not necessary, to read the sub-series in order. The only books that really need to be read together in order are the first two, The Color of Magic and The Light Fantastic. That’s not only because they’re stylistically different from the rest of the series but because they’re really two volumes of the same story. The Color of Magic ends with a literal cliff-hanger that is resolved at the beginning of The Light Fantastic.
But you can worry about reading those later. First things first. The Fifth Element comes very late in the Sam Vimes/Ankh-Morpork City Watch sub-series, while Reaper Man comes fairly early in the Rincewind/Wizard sub-series. It also prominently features Death (that’s the Death, you know, the Grim Reaper), the only character who appears in all the books. So you’re probably better off starting with Reaper Man although personally I think The Fifth Elephant is the better book.