Instead of just asking which Kontakt format library is the best, which will be nothing but an opinion, please tell me for each of these instruments what you have been very satisfied or dissatisfied with (It may be from an all purpose set or an instrument specific set) I will include with the list some of my feelings:
Flutes: Sounds fine on VSL but had trouble get legato and vibrato right
Bassoon: Very unhappy on VSL (Kontakt 2 Library)--harsh attack
Harp: Giga Harp was great but haven't seen a K2 version. VSL isn't bad
Trumpets: Never heard anything exceptionally good or bad
Other Brass: VSL satisfactory.
Strings: Very hard to get realistic sounding. I heard some nice Garritan demos but demos can be deceiving. The Reason Orchester has better strings than one would expect, but there is no keyswitching available.
Timpani and Percussion: The best I've heard was an Ultimate Orchestral Percussion for giga converted to K2
Upright Bass: Never found anything useable.
Organ: Kontakt Surround Organ is okay. Didn't like the other PMI organ on K2 Discs
Thanks for suggestions and also feel free to give me advice on whether Garritan Big Band is worth the money if only interested for the brass. I really need a nice muted jazz trumpet with controllable vibrato, legato and articulations. I wish I could play the real thing.