Well, I'm having a little trouble with the first two myself (I don't read a whole lot of first-person novels, so I don't have any go-to authors in those categories) but C.J. Cherryh is a pretty reliable bet for #25 and Lois McMaster Bujold is excellent for #27.
Cherryh tends to be a bit dense and can be slow going, but she is always worthwhile. I strongly recommend The Faded Sun Trilogy for this challenge for several reasons. First, it's pretty commonly available in stores in an omnibus edition; second, each book in the trilogy can fit in a different category (say the first book counts for #25, then the second book can count for the non-generation ship category, and the third book can count for the work set in a human intergalactic empire category OR in the work in a universe with multiple non-human intelligences in contact with humans category. And any of them can also count in the third-person limited, multi-perspective viewpoint category. I think it is also some of her best work, most characteristic work -- it features several of her common themes (first-contact, though the books don't QUITE fit in the first contact category, because it's several decades after the first-contact occurs; the importance of language in shaping thought; culture-clash; it also features fairly typical Cherryh alien races -- distinct from humans in their psychology though without a ton of emphasis on the biology; and the climax of the trilogy is really, really impressive. I will admit, there isn't much resolution in the first or second books, but because you'll probably be buying the omnibus edition and each book is at the short end (I think the whole omnibus is only 700 pages) it isn't quite the commitment of a longer series.
Bujold is pure delight -- her Vorkosigan series is in my opinion some of the best science fiction ever written, and that is attested by the impressive collection of awards it's received: four Hugos, three Locus SF awards, and two Nebulas, as well as multiple other nominations for all those awards. The thing that's even better about the series is that each novel is complete in itself, and while I don't recommend people read a book late in the series before an early on, it is certainly possible to only read book one. For even greater convenience, it even has two potential starting point to the series proper, and two related books that can be read entirely on their own. The two starting points are Shards of Honor (which is usually found in the omnibus Cordelia's Honor) and The Warrior's Apprentice (which is usually found in the omnibus Young Miles); and if you get either omnibus, again, the second book can easily count in another category. So say you start with Cordelia's Honor, the first book can count for the third-person limited, multi-perspective viewpoint, and the second book (Barrayar) can count in the Hugo or Locus Award winner category, or it can count as Space Opera, or it can count as Military SF, or it can count in the human interstellar empire category. And incidentally, the two total stand-alones are called Falling Free (which can be #27, Nebula Award winner, or work set on a permanent man-made space habitat) and Ethan of Athos (which can be #27 or human interstellar empire). Personally, I think the series proper is higher quality than either of those two stand-alones, but I will warn you that it is VERY addictive. ;)
Also, I'd suggest posting this in the "Suggestions" thread in the SF Challenge group as well. . . you might get more answers there.
