Being the one I've played the most of, I would of course recommend DMC. It's also the one with the least amount of games in the series (only 5) along with a 12 - 13 episode anime so it's not a big ask to get into it. All the games are fairly short, although they do have several difficulties you have to unlock and they're pretty fucking hard (especially 3). DMC 2 is a bit of a rotten egg in terms of gameplay, although at least the story is fine for a single playthrough. DMC 3 is easily the best in the series in terms of both gameplay and story. Oh and avoid the reboot (DmC), the reboot sucks. The gameplay's not bad but the story and characters are complete crap. Chronologically the games go 3, 1, 2, 4 and 5 but I'd recommend just playing them in release order.