A few days ago, I read Erik Larsen's column about Nestor Redondo
http://comicbookresources.com/columns/?column=20After tracking down a few Rima issues, I got to thinking about other artists lost over time. Please, feel free to post any overlooked artists (you have a soft spot for).
Right now, I'm really into Alex Raymond's Flash Gordon serials. I can't get over his fab work.
I had a quick check (without delving too deeply into the comics boxes) -- and I knew I had some Nestor Redondo comics reproduced in black and white:

I'm not sure you could call Alex Raymond undervalued. Could there possibly be a good comics artist anywhere who doesn't understand the influence of Mr. Raymond?
I do prefer the European artists to the American when it comes to the comics that aren't mainstream, or "super-hero". There are some in my collection I could name, but I doubt you would have heard of them. When I can dig out the mags I could post some images of fantastic artists. I especially like "good girl art".
As mentioned in the column, I like Alex Nino's art. He's great on SF or quirky horror tales.
One of the best comic book artists of all time was Mac Raboy. Again, although he is not so well-known now (as far as the modern comic book buyer is concerned) it would be a stretch to call him "undervalued". I don't have the book to hand, but if anyone has the superb Steranko "History of Comics" Vol. 2 (I'm sure it's Vol. 2), there is quite a bit of information about Mr Raboy, from what I remember. I suppose he is like Mr Raymond in that he was a very skilled draughtsman.