Hmmm where to start, now that's a good question, my normal answer for it would be, what feels the easiest for you, I tend to use pixel art quite often since it's relatively fast to make, I am also somewhat decent at vector art too and know a thing or two about digital painting

I feel it's less of a what style to learn and more of a what software to use however, I personally can't make many reccomendations, mainly due to mostly/only using a certain one which I have used ever since I started since it is what feels the most comfortable to me, which also many I know don't feel comfortable using, it's even discontinued (good ol fireworks 8) so welp there's that, I know and have used some others though

From my experience, Pixel art is really easy to get into, and make somewhat decent works with some time spent in, hard to master however, I have practiced it for many years and there's stuff that surprises me every now and then, and I feel my abilities are still lacking quite often, Vector art might feel intimidating at first, but it's actually not as hard as it seems, that part is more of "how skilled are you at handling the program you are using" for me, if you are fairly good with it, it's not going to be very hard, since you can constantly change your traces and stuff, digital painting is... depends on the tools you have at hand and your personal skills, voxel art is about as easy as pixel art, probably easier to master, 3D modeling has been a nightmare to get into so I won't speak much of it, it's probably more of a case where I still haven't found the program I can be comfortable with for it