I still think Serum is one of the most useful ITB synths ever created. It’s got everything I need for general work and sound manipulation, and it’s laid out in a sensible way.
Omnisphere is incredibly handy for sound design and manipulation, though it’s a huge, lumbering beast that can be hard to work with.
Waldorf’s Largo has wonderful oscillators. This one of those situations where a big sonic improvement in one area makes up for any other shortcomings.
Valhalla: sounds pretty, easy to use, lots of options but not overwhelming, laid out in a reasonable way.
FabFilter: Sonic quality and easy of use. I know it’s “mix with your ears, not your eyes”, but the real-time frequency spectrum is handy for seeing what you’re doing. And plenty of little tweaks under the hood to really customize basic effects.
u-He: I consider the Uhbik collection to be the gold standard for effects. Everything works like you expect it to and sounds amazing, but has a real depth to the controls and results. A, D, and G get a lot of use on my tracks. Zebra is a lot of fun for both synthing and processing. Also, Satin. Seriously, nothing they put out is bad.
SoundToys: Decapitator, especially. Great for everything from a warm up to overblown distortion, and models some of the great analog overdrives out there. Again, all their stuff is top notch.
Small batch/one off/interesting:
Audiority Grainspace - wonderful grain delay with lots of options. Sounds lush and fun to ‘play’ in real time.
GlitchMachines: Convex, Fracture and Cataract are great examples of interesting, useful and unique plugins that do things you don’t find in most ‘big name’ VSTs.
Ohmforce Ohmicide: The most ridiculous, full featured, creative distortion plugin I’ve ever seen.
New Sonic Arts Granite: Another crazy fun grain plugin, this time a sample mangler/wrangler. Can go from lush to weird with a couple of clicks, and very automatable.
Eventide Blackhole and Mangleverb: All of Eventide’s plugins are amazing, but these stand out as making delay and reverb interesting and fun. Like an H3k without all the fiddly bits.
Dron-e (Reaktor): So many amazing Reaktor ensembles, but Dron-e stands out as interesting, full-featured and playable.
LFO (M4L): Yeah, it’s simple, but being able to LFO just about parameter in Ableton or a VST is huge.