Hm, this is a tough one for me. Both guys are ridiculously similar in a lot of ways. Batista's beaten some of the best of his era, but so has Roman Reigns. While names like HBK, Cena, and the Undertaker may sound more impressive than AJ Styles, Randy Orton, and Daniel Bryan, the fact is that Reigns has never had the opportunity to face the three names listed above (Taker, Cena, HBK), at least not yet. I don't think that proves anything. I can just as easily say that Batista tapped to Daniel Bryan at Wrestlemania and than got pinned by Reigns at Extreme Rules. Reigns has also beaten an unstoppable monster in Braun Strowman and damn near beat a more unstoppable Brock Lesnar at Wrestlemania, so this argument isn't a dealbreaker for me.
Both guys were the muscle in two of the more memorable factions in the last 20 years. Batista was more of a bit-player, playing third fiddle to HHH and Orton until 2005 though while I viewed The Shield as a much more equal unit. Still when he broke out, Batista broke out big. There was nobody hotter than Batista in 05-06 except maybe that one guy John Cena. Roman Reigns never really distinguished himself as special, the only thing that's done that is his booking.
Batista won 6 World titles in a span of 5 years. Reigns has won 3 in the span of 2 years. Batista has clearly had the more memorable championship reigns in my opinion. Yup this one's tough. I really have no idea who would win a match between these two if they were to face off in their primes. They'd probably split more often than naught as Cena/Batista did.
I'm probably going to go with Batista because at this point he's reached greater heights at his peak than Roman has. Reigns' peak has yet to come and in maybe a year or so this would be a different story, but right now I'm probably voting Batista.