A lot of great suggestions here. My goto's so far are:
Bass- Ilya Efimov: Classic Bass and Modern Bass. If Mondo was around when I was looking for bass it looks like it might be all you need.
ACoustic Guitar- Ample Sound: Nylon Strings, Acoustic Samples:Sunburst, NI:Session Guitar 1&2 (I will be picking up Electric Sunburst)
Electric Guitar- Ample Sound G, Impact Soundworks Shreddage 2 or any of the Shreddage offerings.
Amp Sims- Le Pou is great for free and very much in line with commercial offerings. Bias Amp&FX is my got to but plan to get Scuffham S-gear. Ni Guitar Rig for more crazy effects.
Drums- Addictive Drums 2 and EZdrummer 2, Battery 4, Izotope Breaktweaker. Sample Logic Havoc (Heavyocity Damage would be similar). Soundiron Ape
Pianos- I use the pianos in Komplete mostly. I like EZkeys, but I don't like the sound of the piano as much as the others so if I use EZkeys to help me construct a piano part I end up replacing it with either NI's Giant (because it's so versatile) or if that doens't work I start trying out the others. Also I like some of the pianos in Sampletank 3 but ST3 doesn't always play well with Kontakt for me in Reaper so I try to stick to Kontakt.
Organs: NI's Vintage organs has some great stuff in it and so does UVI's retro organ suite.
Synths- If I had Omnisphere I'm sure that's all you'd need but I don't so here is what I use- Absynth, Massive, FM8, and Razor. Monark is great, I just haven't actually used it yet.
Strings: Lass (mostly the divisi sections and the First chairs)
Others: Output Rev,Output Signal, Output Analog Strings, Kontakt Factory Library- Mostly the sampled synths. (Starting to use Exhale- not sold yet on the whole vocal chops trend though so I'm trying to see how I can use it differently)
Edit: I forgot to add in a Solo String. Tina Guo is a must when on sale. Still searching for the rest. So far I like the sound of Embertone's new offerings as well as Cinesamples, Virharmonic and Chris Heins. But I think you said you were pretty well aware of string libraries so you probably have your own favorites
Horns- I haven't actually used them myself yet but I think NI's Session Horns pro looks like a great product for this. Also, as mentioned Sample Modeling stuff looks great for this as well, probably a little more work but as is usually the case probably a better payoff as well.
I'm finding that sampled acoustic guitars benefit greatly from some tape machine VST's and some saturation(dirt) as well as a little eq and reverb, of course. I also bounce them to mono. I find that a lot of the stereo effect built in, especially with NI's strummed acoustics gets phasey quite easily so I just bounce to mono, double the track pan left and right and create my own doubled effect or if I want a fake stereo effect if I use Plugin Alliance BX stereomaker, but you can also experiment doing this with delay for this as well, which I've done as well. BTW the bx stereomaker is on sale for $29 right now. Great plugin.