The full Corel Painter 2021, currently with a healthy 45% discount and seemingly not a subscription…
It’s still a contender for natural media, since no other software yet provides a full ‘all in one’ natural-media painting software with fast brushes on a desktop PC. Best for those who want the full range of paint and ink types in one well-developed software, and who intend to get heavy with oils, impasto, textures etc. Painter 2019 could still load and replay Poser Sketch scripts, and I assume 2021 can also. Bear in mind it’s now “Windows 10 only” (ugh). Corel Painter 2020 seems to have been the last version to support older Windows, back to Windows 7.
The alternatives:
SAI 2 has incredible speed for speed-painting with big blending brushes on a big 7000px canvas, but… an antique UI that even lacks basic things such as a ‘dark mode’.
Paintstorm is not far behind SAI 2 in big-brush speed and has lovely blending. Its sister software Realistic Paint Studio does much the same, and has a charming UI that will appeal to many. See the recent review for the latter, in Digital Art Live magazine.
Krita is free and now sort-of has watercolour and oils, and Krita 5 may or may not bring the full caboodle in that respect. But aspects of the UI such as brush-handling, and especially the obtuse naming conventions, are difficult to love — especially if you’re coming from Photoshop.
Rebelle 4 has recently added realistic oils to its superb watercolour effects. Great if you’re scanning in lineart and then adding digital watercolour, but it is expensive for what it is and almost never discounts. But it’s so realistic that you basically need to learn to paint in watercolour like you would in real-life. Which is not as easy as it sounds. The software also develops very slowly.
Clip Studio is good for inking line-art, but it’s horribly complex and fiddly and that will constantly get in the way of your creativity. There are now inking options that are just-as-good. The $20 Paintstorm, for instance, does very nice inking, and also has adjustable gap-fill on colouring line-art.