Devil May Cry 1 is the best in terms of Manly voice acting and epic sound effects (in my opinion). Seriously, I can never tire of the stinger sound effect with Dante's shout. That is just epic. Plus the awesome, cruel and scaringly design of the enemies.
Devil May Cry 2... I have to get farther in the game but so far it's the best when it's about no story cutscenes for a big part of the game.
Devil May Cry 3 for a great soundtrack, quick combat system, epic boss fights, etc.
Devil May Cry 4 for the option of having two selectable characters (I know DMC2 has that too but I haven't tried everything yet and I can't say that much about it yet) whom I like to play as both. It has a quicker stylish system, it has style switch making Dante less 1-sided, it has the best bosses in my opinion (Every boss excluding Agnus in mission 6 and both Sanctus battles imo). And... I've grown to like DMC4 mostly over time so, in the end it is the best in my opinion.