Few days after May madness is good timing I think to relaunch this post.
With default instruments out of scope :
1 - Friktion : I don't like strings very much but this is a class on its own. Its guitars and flutes are from another world of realism and with so much parameters, realistically changing the sound, it is an unequalled expressivity to me, Physical modeling showing its superiority against hyper sampling (in particular for this kind of instruments). Resonans is a nice try on physical modeling but Friktion is an home run imho.
2 - ReSpire : I may be the only one to like (love) this RE and I understand the VST may be better. But oh my, these sounds, these presets are just so powerful, they just need to be tuned to your track. Even browsing to serum presets is not as satisfying and fun.
3 - Antidote : Awesome presets, awesome usability, decent UI. Maybe my most well rounded RE.
4 - Expanse : very nice controls and kind of intuitive workflow too, excellent presets.. And I do like the Ui.
5 - Oberon - Algoritm (tie). I like both equally and I have hard time to create the sounds I want with them equally
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(very) honorable mention: predatorRe, The Legend, Parsec, Scenic, Go2
To a lesser extend complex-1 and Vk2, modular may not be for me...
Bitwig and RRP fanboy...