Love all of them but it has to be Aoba... he's the only one where I came out liking the protagonist more than the capture targets by a long shot; not because they're bad by any means, I just find him so charming and lovely! I have yet to hop into Slow Damage and Clean Dishes though, so subject is open to changing in the future; plus I've been in the mood to do a reread of their whole release catalog anyways, who knows how that could go.
Also, not a protagonist, also, but they sometimes include him in the protagonist merch lines: shout out to Naitou-kun, love that guy, would love to know more about him like we'd gotten to with Sonico (and to a lesser degree, Ouka) in the Nitroplus side of things.