I like to try out different programs, so I have a lot to say about several, including some more obscure ones!
Corel Painter is my go-to (I use the 2021 version) not because it's the best one, strictly speaking, but because I like the interface and I think it has a well-balanced suite of features. In my experience it has less lag than PhotoShop and better brushes, but is otherwise quite similar when it comes to drawing - it isn't a great photo-editing app, though. I would recommend that nobody pay the full price for Corel Painter because it is too expensive for the features you get. However, once a year, they put the previous year's version of Corel Painter on sale on Humble Bundle for like 20 bucks, and then it's a very good deal and supports charity.
Rebelle Painter is another program that goes on Humble Bundle for a low price once a year,
and it's actually on there right now if you want to check it out. However, unlike Corel, I think Rebelle has a unique enough and impressive enough suite of features that it's even worth the full normal price if want to get it while it's not on HB and if you can afford it. It's less versatile than a lot of other digital art programs because it's very dedicated to emulating traditional media, but it is the best at what it does by far. It has very realistic emulation of paper textures, watercolor, oil, pastels, and so on. The markers aren't very realistic (digital markers are the bane of my existence anyway... markers are my favorite traditional art medium but NO art program on the market can emulate them worth a shit) BUT they make up for it by being excellent for smooth blended fields of color. Which is not what markers do in real life, but it's fun. If I was going to recommend an art program this would be my
#1 recommendation, unless the person preferred to make art with a digital LOOK to it (which Rebelle is kind of bad at). The realistic pigment blending in the pro version is especially nice and IMO worth getting the pro version for.
Clip Studio Paint is a better and cheaper PhotoShop replacement for digital art, and that's about it, IMO. It does have a really good fill bucket tool, so it's great if you're doing flat colors. I'm not pleased with their move towards a subscription model, though, and I don't like the brushes as much as I like the brushes in Corel and Rebelle. I got a brush bundle I really like for CSP from True Grit Texture Supply a while ago, though.
PhotoShop itself is ass. It's only really top of its field for photo editing and not art in my opinion, and even for photo editing it lags and is therefore not the BEST on the market. Don't buy PhotoShop unless your school or workplace requires you to do so.
What is the best on the market for photo editing? Well, I don't know, I'm not a photographer. However, Affinity Photo is definitely just as good as PhotoShop in that it seems to do all the exact same things while also lagging less and being a cheap one-time purchase instead of an extortionate subscription. It's kind of mediocre for drawing, however, even though it's great for photo manipulation. Like, it's not bad, but it's clearly made with photography in mind moreso than drawing. Affinity also has 2 other programs, Affinity Publisher (which is like InDesign and not really an art program, but great at what it does) and Affinity Designer, which is my favorite vector based art program that I've tried by a long shot, but take that with a grain of salt since the only other vector program I have tried is Adobe Illustrator and. well. it's Adobe. I have a soft spot for the Affinity suite even though I don't use it as often since it's better suited for a different style of work than I do because I think it's probably the best positioned software to challenge Adobe's stupid monopoly.
Krita, Paint Tool Sai, and FireAlpaca are all programs I used to use a lot but don't really anymore. Sai and FireAlpaca are both awesome basic art programs but lack the bells and whistles I like from some other programs, Krita has some of the bells and whistles but is a little hard to use - the reason I stopped using it was because the colors broke somehow and I couldn't figure out how to fix it despite hours of searching online and trial and error. Krita and FireAlpaca both have the important distinction of being free.
I was not pleased with Autodesk Sketchbook or GIMP, I found both frustrating and difficult to use. YMMV though, I've got friends who swear by GIMP. I think they're both free (GIMP definitely is, but I'm not sure if Sketchbook still is or if that was just a promo) so it couldn't hurt to try them out anyway if you're experimenting.