Keys: Keyscape (I don't have it in real life BECAUSE in real life I am allowed more than 5 libraries, so I have piano's and EP's/synths in abundance. But if I could really have only one Keys-library, I think it would be foolish not to take keyscape, for its sheer count of keys - it's almost cheating). If I am only allowed to choose from the libs I do have, I think I will take Soniccouture's The Canterbury Suitcase with me.
Drums: Superior Drummer 3. Who could ever need more?
Strings: Spitfire Chamber Strings (pro). I have the core version, but would have plenty of time on my island to experiment with all the different mic positions the pro version offers.
Bass: Modwheel's The Lowdown has so many options for letting the air move and the sand rumble. You could problaby make an album with this one alone.
Extra: Spitfire's Sounddust Volume 2. This excellent collection of 5 Sounddust Libraries gives me a loopbased, Mellotronish thingy (Flutter Dust), a quircky piano lib to spice up Keyscape (Ships Piano), a quircky drum/groove lib to spice up SD3 (OddHop), a unique guitar based interument (Pendleonium) and a library no-one at a desert island should live without, due to its name at least: Infundibulum, which is indeed an infinite sonic playground.
Where can I buy my ticket?