Well, Kingkiller is not a trilogy at this point, and while I've remained cautiously optimistic about both Martin and Lynch completing their series (or in Martin's case at least Winds), even I don't really believe we'll ever get a next Rothfuss instalment. The first book was very enjoyable, though perhaps a tad overhyped. Good prose, interesting setting, some character and pacing issues but overall good. The 2nd book had serious issues, though. I'm not saying don't read them, but I would not say they're essential or anything.
I have not yet read Mistborn or Fitz, so I can't really comment on those.
I did complete Stormlight Archives (the 3 that have been published, at least - though more are planned, so not exactly a trilogy either!). Many people recommend starting with Mistborn, I didn't have any problems with continuity/ understanding Sanderson's universe. There were some things that hinted at things probably described in his other books (mistborn?) later on (mostly in Oathbringer), but I think starting with Stormlight is very much fine. It's considered his best/ most mature work, I believe.
They were very easy, quick and enjoyable reads for me, which is quite impressive when you look at the page count. I don't think they're great, however. They sometimes get a bit too YA for me, and the characters nor prose are comparable to the best in the genre. Definitely worth reading if you have the time, however.
Out of those I've read, I would pick The First Law. What is there to say? They're really good books. I'll still offer some criticisms, since that's what I've done for the others. I wouldn't usually compare a series with what I consider the Gold Standard, but it seems fair here, as Abercrombie obviously was very much inspired by ASOIAF (and yes, I still consider that the gold standard - go away tv show!). The First Law, in my opinion, is "lighter". That may seem a strange things to say about a trilogy featuring torture and horror as main components, but that is how it felt like to me. Not due to the subject matter, but because of the writing. ASOIAF is quite somber, but it's very clear that - while putting them through some horrible situations - Martin still wants you to root for his characters. Abercrombie on the other hands espouses a very nihilistic view in his books. This, while providing for very entertaining characters and books, makes me care somewhat less for them. I love reading about them, but I do not think Abercrombie would be able to practically break my heart the way Martin did. If I can add another slight criticism to this... At some points his characters come close to being caricatures/ slightly cartoonish. Jezal's vanity, Ferro's anger (which seems to be her whole character), those 2 rival Union generals, the prince whose name I forgot... To end on a positive note (and I do truly believe this is a great series), Abercrombie is still improving. I believe the standalones are actually better than the trilogy, and am looking forward to reading A Little Hatred.
I don't think there's necessarily a wrong choice, though.
