Depends on the situation. I always want capacity that I can depend on without worries, and have a preference towards plastic backed with tapes. For wearing under clothes while out and about I need something comfortable and thin enough to fit inside my jeans. At home I like thick with cute prints! I'm also picky about plastic like @Subtlerustle . I usually wear Rearz and what I like is that even different prints in the same diaper type use different plastics, so each print is a different experience! Safari has the perfect feel to me .
I have all kinds of diapers in my collection and I will wear each and every one of them. But as for preference I like hook a loop fasteners , high capacity , and tall leak guards so I can use a booster or two
I dunno. What's the diaper equivalent of fahrvergnügen? LOL.
But I'm kind of serious, actually. I'm always tempted to answer questions like this with things like "babyish" and "feminine," and indeed those are contributing factors, but there also plenty of diapers that are those things and yet hold zero appeal (for me). As well, there are things like prefolds that are strongly babyish to me because of my particular childhood experiences, but aren't innately babyish in ways younger people can easily relate to. Basically, it all comes down to: I'll know it when I see/wear/use it.
Despite that, my preferences are kind of all over the place, spanning disposables and all varieties of cloth diapers, so in practice I'm not exactly hard to please. It would probably be simpler to list things that are complete turn-offs for me, and there's really only one: Anything overtly medical in appearance.