I think the most beautiful place we've ever visited was Lake Bled, Slovenia. We added it to our 3-week road trip last year as an addition to fill a few days, we didn't know what to expect, but on arriving we were totally surprised and in awe of the beauty.
It wasn't just the lake that we thought was beautiful, but the surrounding area was breathtaking. Definitely worth a visit.
We'll be returning to take in more of the country as I don't think we'll be disappointed.