When I think about which artist's work upon seeing it has changed my preconceptions about it, I have to answer that, other than the old masters, new work has impacted me usually in passing and great works of artists now unknown to me have touched me. Ultimately there are handfuls of those whose work grew in stature and importance after actually seeing it for real. Maybe Jenny Seville would be an example. Stepping back, an artist whose work really impressed me is Goya, but that really comes from knowing the context of his life and history, what kind of establishment he was working in, etc. In pure painting, I spent a lot of time in Prado in Madrid and Valezquez, Murillo, Raphael, Brueghel, and Botticelli in particular impressed me, looking far better in person or being even more dynamic than I had imagined.
When it comes to modern art, many of my experiences in viewing the actual work has been somewhat disenchanting. Guston is an exception that comes to mind, but often something about the quality of the painting whether it be how the artist handles surface, deterioration and imperfection in preparing the canvas, or simply the flawed human presence in the work that's not so apparent in an image in a book, is uncomfortably demystified in a gallery, which might only be a result of finding that the wizard of oz is actually a small man. What has impacted me in a more powerful way is being familiar with an artist's work, and then, in the process of looking, learning the context of the work, the relevance of the work to the artist, and then, bang, the work opens wide for me and takes on a very magical quality. This happened most dramatically for me with Mondrian. He is an exacting and technically proficient, eloquent painter, but to know how he came to paint his squares and what they meant for him, his adherence to theosophy and his search for a universal language is to truly begin looking at his work. At least it was for me. But I'm not so sure that his philosophical theories were contained in his paintings as much as they were illustrated by them.