Here's where I've been:
create your own visited states map
Wow, almost all of them! I hit the majority of them in three trips: a family train trip to Montana when we were kids (My parents are that 0.0002% of the population who enjoy train travel. I, however, do not.), a post-college road trip from Boston to New Orleans, which was my first big adventure south of Mason-Dixon, and my drive from Boston to San Diego last January. And yes, I insisted on a detour to Four Corners, because when I was a kid memorizing state capitals I thought it was SO COOL that you could be simultaneously be in four states at once, when in fact it is probably the most bleak, isolated, and boring national monument in the country (although I did lie down in the middle of four states and that was pretty cool).
I'm just missing a few in the central west part of the country, and I can't remember driving through West Virginia, although I'm fairly certain I must have at some point.