First of all: Logic. I use it since Atari days in the 80ies when it was still called Notator.
(Funny thing is that i paid for lots of its parts at the time when they were new and not integrated yet, such as the ES2, the clavinet and the hammond emulation.)
Second: The VSL cube. i preordered it around 2004 (or 2005) and the woodwinds are still my goto. Not because i wouldn't have others, but in an orchestral context they just work extremely well.
Third: Zebra 2. I was an early adapter on that one, when it was a 1 person company. I remember when once i had a problem and sent an email to u-he. About one hour later Urs mailed me a new build. At around 2 o'clock AM.
4th: Ozone. I bought it first when it was at version 3. It's still in every single project (in most cases only the imager and the Limiter and the dither module).
5th: izotope's RX: I record a lot of stuff and RX makes my life so much easier...
6th: "Workhorse category": Kontakt, Echoboy
Extra category 1: Plug ins with a fantastic RoI that i don't use that much anymore: the Sonalksis Compressor, Waves S1 (still in almost every project, however nowadays quite often replaced by Precedence), the Exponential Audio reverbs (still get lots of use, but the Precedence&Breeze combo, Liquidsonics CRP, Sonsig do lots of the actual heavy lifting)
Extra category 2: More recent purchases: Breeze & Precedence, TDR Nova, lots of PiA stuff.
Extra category 3: Real instruments: My upright Piano. While i mostly use sampled pianos in my projects, I still practise (a bit) every day, to keep my level. However, 3 years ago i turned it into a prepared piano for 3 months and used it as a signature sound for a film score.