Ill start.
Arturia Pigments, it seems like it should be so good but i find the filters really flat sounding and it feels like it takes a lot of work to find good sweetspots when designing a sound. The sweet spots also sound really “vanilla” in my humble opinion.
I would say that Surge XT, which is free, is noticably nicer sounding and more fun to work with.
How about anyone else?
I don’t go in much for regret, but basically all of them except the ones I actually get some use out of. Namely Serum. I just don’t use plugs much. It’s more fun to me to do as much as I can w the native DSP. I appreciate the simple GUI and consistent controls, and the general semi-modular nature of renoise
I basically stopped buying plugins some years back, because I would drop coin on the current hype, not use it, and keep turning to native DSP.
I have enough VSTs but don’t regret any of them only I probably shouldn’t have bought Softube Model 72 and Model 84 had I known I’d get Arturia V collection which include both emulations, but the Softube plugins are excellent. Pigments is currently my most used instrument 
Always trial before buying and you won’t end up with regrets. 
edit: Actually I regret buying Exponential Audio reverbs, even though they sound great I use NI Raum and Fabfilters reverb instead.
No regrets, being honest. I just buy stuff when I’m 100% stuff I’ll use them.
I think my least useful purchase was Vybz, because I already had Infiltrator 2 (which can do the same stuff except for the tape texture thingies). Luckily, I only paid $10 in a sale so no regrets (Infiltrator was like $70).
About Pigments: It’s great but too “over the top” in my opinion. CPU usage is also horrendous considering what you can do with Phaseplant without barely loading your processor. I’d take Surge XT over Pigments any day (maybe with the exception of the granular engine - but Phaseplant has one).
if i’m being honest, redux, lol. i hesitated between buying redux and renoise and ended up only using redux long enough to learn the tracker interface and immediately converted to renoise. maybe one day i’ll get into using redux in renoise.
I feel the same way @eight_trax, I never use Redux but I’m still glad I supported the developers
But, I have to say the kickstart compressor and wait for it….Phase Plant.
I now use automation for the rhythmic pump effect with native fx and for a tempo synced side chained kick and bass…a little math never hurt lol. Kickstart 2 is outta here
Phase Plant just comes bundled with a bunch of FX plugs that I already have natively and it’s like another sampler fx tab within Renoise. It even has 8 macros too 
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I’m WAY more interested in sound design with the samples
So right now it’s just Kontakt and Arcade and I render those puppies to xnris
Although I use kontakt, I am a bit regretted buying the whole Komplete Ultimate 12 because guitar rig, kontakt, Action strikes, giant, and Grandeur are the one I actually used from the bundle. I got the Ultimate because I wanted to have some orchestral instruments, but it turned out that the Symphony Essentials was a total disaster (Especially the horn ensemble), ended up a total waste of money and additional library were needed.
Edit: It is not about vst though, but worth to mention.
I don’t “regret” buying whatever, because I always think before I buy on a Black Friday and I don’t buy stuff I don’t need or probably will never use. But there are a couple of VSTs I haven’t used yet, such as
Nevertheless I don’t regret buying them, I didn’t spend that much because I only buy on Black Friday, and I will probably use them at some time in the future. I knew that I won’t use Transit that often. But I’m a little annoyed by the fact that the Softube Compressor isn’t no more, now there’s a successor called FET Compressor MKII. I could “upgrade” for 29 €, but what for? I even didn’t use v1 until today and I’ve spent almost 100 € for that stuff. I also could have saved money if I just knew that Phoscyon was about to get a successor as well, I think that I bought v1 a couple of weeke before v2 was released. So yeah, I should make a new shopping rule: “Don’t buy stuff that was released a couple of years ago!”
I’ll only buy Redux if it gets Midi outs 
Otherwise I’ll probably regret it.
i used kontakt a bit way back in the day and found it to be a great alternative to the rack samplers of the time (im talking kontakt 1/2) but i felt NI took kontakt towards a direction more for scripting epic libraries etc than a super sampler. you’re using the newer versions to do sampler work and translating it to renoise?
Yeah I have the Komplete 14 bundle. It comes with a few synths too. But yeah I render them to xnri unless the bank is very dynamic like guitar rig, then I render my melody to a sample. Sustained instruments, plucks, mallets, instrument phrases, etc work better in Renoise’s sampler imo.
Kontakt’s wav sampler is trash and so is Arcade’s. When I’m not making beats I’ll load them up, find sounds I like and render them to xnri. That’s how I build my library, oh yeah and using instrument players from TikTok too 
Do you remember Intakt? Now that’s old school
For me it is Icarus 2. It is a very good synth, and having a lot of features. But I have enough wavetable synths. So I never use it. I also cannot sell the license, it was NFR.
This is why I tend to avoid getting plugins having overlapped functionality. Even if they have one particular unique functions, I know I would ended up sticking with the one I owned earlier because I had put the time learning it.
I learnt the lesson on that Komplete bundle I have mentioned before. I know Reaktor is a much more powerful modular synth, but I always sticking back to SunVox simply because I know it much more than Reaktor, thus easier for me to make good sounds.
Softube Modular is a pretty big hunk of shit, so I kind of regret that one. It was only $50 on sale, but the polyphony handling is horrible, you have to drag out an entirely new oscillator, EG, amp, etc per voice and the stupid interface macros never seem to actually work or map any differently than just 1:1 on whatever knob you assign. Maybe there’s some secret to the macro modules but I haven’t figured it out.
They also nickel-and-dime you for upgrades harder than everyone else. Typically you can get a pack of like 30-50 modules for $50-ish on another platform, and for the same price you can literally get just an oscillator via Softube. Not sure why I didn’t see these red flags prior, but that’s on me I guess.
Multiphonics CV-1/CV-2 is a close runner-up. Charging an extra $75 for bug fixes and basic functionality (while basically being the same damn thing, in a nutshell) is just cruel. I figured I was getting more or something, but that was my dumbass fault for sure
I bought Unfiltered Audio’s Silo, BYOME and Spec Ops recently. There is too much overlap between BYOME & Spec Ops. But with the discount I got both for the price of one. So it isn’t too bad. And while I love the idea of Silo, I can’t seem to make it work with the music I’m making. I do like the (crappy) sound of their reverb. But those same algorhythms are in BYOME & Spec Ops.
But the one I regret most is Spitfire Audio’s Abbey Road. That one just isn’t for me.
That’s how it works in Modular Land, though. So in that sense it’s an accurate emulation of the hardware modular experience. If you’re not up for that, use a polysynth.
Damn, I’m spoiled to VCV, VM and Poly Grid where you can just mono-to-poly your voices and vice versa. It saves so much space, so I’d probably never want hardware
Oh man, BYOME, TRIAD and SpecOps are probably my favorite VST FX in the world. Which features do you find overlap so heavily between SpecOps and BYOME? TRIAD is a hell of a lot closer since it’s just kind of BYOME x3, but I always felt like SpecOps was like its own unique spectral processor on the side
I don’t use it anymore but TRIAD is pretty awesome and I used it heavily before I discovered Renoise. Had BYOME first though so I definitely wasted money there lol. If you have TRIAD you really don’t need anything else from UA they all use the same algo. But they do have a great sound.
I can make my own multiband FX chain now with WAY more flexibility. I did love that reverb in TRIAD and the pitch delay, but even that can be recreated in the FX tab 
Also with Pigments, the envelopes are also not so good. The release of the envs don’t have curve controls. Only attack and decay have adjustable curves.
Yes. Also they also have a brutal click when the attack is at zero, even when modulating filter cutoff. Not sure why its so bad.
Regret is a big word, but this thread reminds me to put more work/time into the reason rack plugin. It has a ton of options, I just have a hard time reading the gui, the controlls are small. Perhaps there is a vst3 version now that can be scaled?