Good topic, and a nice one because there are no wrong answers.
I was glad to see the OP mention Rien Poortvliet. I actually met him when I was young lieutenant back in the late seventies. He was visiting Germany, and a German hunting companion invited me to lunch to meet him. I don't pretend to be much of an artist, but he affected my personal style more than any other. Literally haven't picked up a brush in decades, but in the early eighties, selling a few made a big difference to a young captain, his wife, and two little ones.
Two of my water colors from the early eighties channeling a lot of Poortvliet.
I won't list any of the masters, we enjoy those at museums. But I will list some that we have found and could afford over the years. Ella Koepke Mewhinney was a Texas colorist whose most notable work was done before and slightly after WWII. She created both wonderful impressionistic landscapes and still-lifes. We are fortunate to have three, and these are the two favorites representing both styles.
Because of my military career, I have always been fascinated with art that the subject of warfare has generated. I have probably a dozen but these are my three favorites. The first is a Stivers original oil portrait. I have a number of his prints, but it is special to have one of his originals. Those Southerners reading this will immediately recognize John Moseby - the Gray Ghost of the Confederacy.
The other two are by Allen Egnell. A mid-twentieth century Danish artist, he is not well known in this country. But his military works are superb and superbly accurate usually reflecting 17th and 18th century warfare.