Close tie between DMC 3 and 5 but it'd probably be DMC 5, mainly because the combat and styles in it feels the most fleshed out compared to the rest of the series personally... sucks that Lady and Trish aren't playable in 5 tho, I loved playing as them in 4.
(and also because there's no freestyle mode in DMC 3 without using the new crimson mod)
DMC3 is the favourite when it comes to actually playing the game, but DMC1 and 4 both have special places in my heart. 1 has the creepy/mysterious atmosphere and the most creative enemies and locations designs. 4 has the coolest artstyle, better controls and Nero xd