Operator 

Although it may not be immediately apparent and the user interface may be a little challenging to navigate, Operator is a very powerful FM synth that can produce any sound the techno heart wishes. Especially metallic sounding FM sounds (what a surprise) can be created with operator. There are almost no sound design limitations once you get past the relatively intimidating Interface. Operator delivers on basslines, kicks, hypnotic synth riffs, hi-hats, and FM plucks. For more complicated patches, practically all parameters can be mapped with LFOs or Max for live devices, allowing Operator to be used as a modular synth.
Especially with the fixed frequency function of the operators, you can construct really complex patches that are difficult to replicate with other synths.
Even though this tip is solely relevant to Ableton users, I couldn’t resist including it because Operator is so strong. At this point, I would suggest Dexed (see below) to those who do not use Ableton.
-> Download Operator

Surge

Surge is a subtractive hybrid synthesizer featuring three oscillators and two sceneries. The oscillators provide 12 oscillator algorithms (Classic, Sine, Wavetable (183), Window, FM2, FM3, String, Twist, Alias, S/H Noise, and Audio-input), with the Classic and Wavetable being the most helpful for me at least. I currently don’t dig the FM algorithms so much. However, the modulation matrix provides a wide range of options that, when combined with clever envelope mapping, can produce incredibly interesting sounds. The Power Horse may provide quite helpful sounds when combined with various filter algorithms. Furthermore, surge has been open source for some time and has a strong community. The Twist oscillator, which is based on the well-known Eurorack module from Mutable instrument, or the OB filter algorithm are two examples of frequently added intriguing new features from the community.
-> Download Surge

Vital

Vital is a fantastic sounding wavetable synth with intricate modulation options, much like Surge. In terms of features, there is no difference between the free and premium versions. Aside from the text to wavetable capability, the free version includes a fully functional wavetable synth, like the paid version. The only variation is in the presets that are offered. For a small fee, you can purchase up to 400 presets to further play with. When compared to Surge, Vital’s interface appears to be a little more sleek, and the modulation animations give the whole thing that special something. Overall, you easily find your way around, and the presets provide immediate results from which you may continue to work.
-> Download Vital

OBX-XD

Many producers use it as well since it sounds excellent. In some people’s opinion, this vst actually sounds superior to its Arturia rival. Although everyone must make their own decisions, those who favor older sounds should look into this VST aufjeden situation. The OBXd, which is based on the vintage Oberheim OB-X from 1979, offers a relatively traditional subtractive synth with two oscillators and a noise generator. The really good-sounding filter is simple to adjust and shines well when used with crunchy, plucky sounds. However, the synth does also a great job producing slightly detuned pads for example.. Personally, I can’t think of anything bad to say about this synth, and I believe everyone should give it a try. Feel free to support the creators with a donation if you like it!
-> Download OB-XD

VCV Rack

Oh boy. Where do I even begin? For nerds and enthusiasts of modular synths, this is an unending playground. Although I don’t think I’ve really grasped the depth and complexity of this fantastic plugin, I still believe it to be the best entry point into the world of modular. VCV Rack, which has a sizable user base, provides an incredible number of modules that are based on actual existing modules. Some manufacturers have also made their modules accessible on VCV Rack in the interim, allowing you to test out modules virtually without having to invest money.
The audio quality is surprisingly good, and it can keep up with some hardware modules. The sole disadvantage is that the software is not yet completely mature, and some glitches do occur. Yet, on the whole, this is reasonable and totally comprehensible given the massive amount of coding work that must have gone into it.
To integrate the free standalone version of VC Rack into your DAW, you must either upgrade to the commercial version or use the open source alternative Cardinal. But as of right now, Cardinal only supports VCV stock modules; it does not support third-party modules.
-> Download VCV Rack

Noise Engineering Freequel Bundle

Noise Engineering is renowned for its iconic eurorack modules, including Basilimus iteritas and Loquelic iteritas. With this bundle, they provide an excellent starting point for those interested in exploring the noise engineering realm. This bundle allows you to experiment with various noise engineering sounds without having to spend a lot of money. Additionally, Noise Engineering offers two exceptional synths – Sinc Vereor and Virt Vereor – to further your exploration.
Sinc Vereor is loosely based on the Eurorack module Sinc Iter and offers an impressive wavemorphing/wavefolding Tone control that simplifies sound design. You can effortlessly blend between familiar waveforms such as saw, triangle, and square. The Noise mode generates self-similar noise for percussion, effects, and other applications, making it even more versatile.
On the other hand, Virt Vereor is a potent synth that comes with three algorithmic modes. The Bass mode, inspired by Bernie Hutchins’ seminal series Electronotes, is a quadrature algorithm. Sawx, on the other hand, is a supersaw-inspired beast. Finally, Harm is an additive algorithm that allows for spectral control and distortion of partials. Together, these modes provide a vast array of sound design possibilities.
In addition to that you also get a distortion plugin: Ruina. It offers an all-in-one distortion. With this plugin, soft sounds can be swiftly transformed into harsh screeching noises. If you don’t want to utterly destroy your sound, use the plugin with caution.
–> Download the Noise Engineering Freequel Bundle

Dexed

If you are not an Ableton Live user, this is an excellent alternative for Ableton’s Operator. Dexed is a cross-platform, open-source FM synthesizer that emulates the original Yamaha DX7. FM synthesis is well-known for its capacity to generate complex, developing sounds. It is also excellent for producing metallic, percussive sounds, which are widely common in techno. Dexed also includes a plethora of modulation settings, allowing you to create changing, dynamic sounds that keep your synth riffs interesting.
Furthermore, because it is compatible with the DX7’s patch library, you have access to a large collection of sounds to get started with. So, why not give Dexed a shot and see what you can come up with?
-> Download Dexed