My favorite artist of all time is Michelango, and my favorite period of art was the Italian High Renaissance. I love the paintings, but also loved the sculptures from that time. I love things that look like they are leaping right out of their mediums and pushing forward right into reality. I loved pieces that were interchangeable -- paintings that looked like 3-D sculptures, and sculptures that looked as if they were paintings.
Please forgive (and correct) me if I'm getting any of this wrong, but I'm going from memory of what I learned in an art class decades ago.
My other favorite artist from this time was Jan van Eyck. I glanced over his Wiki article and was dismayed to learn that he only lived to age 51 (why do so many artists die so soon?!!) And this was like in 1400... which makes his works all the more astonishing.
Mr. Van Eyck was known for his incredible attention to detail.
In this portrait of a married couple, the mirror in the middle features a full reflection of the room -- and, if I'm remembering correctly --
a self-portrait of the artist himself. 
This is a close-up of the mirror in the painting above:
Many of the details later found in his paintings are so tiny, they had to be discovered with a magnifying glass -- and were painted with tools such as an individual animal hair (such as that of a horse or camel, if I remember right.)
Again, I'm working from memory here so I may be getting some of this wrong -- I didn't see this mentioned in the Wiki -- but I do believe I remember my art professor talking about this (and it was details like this that kept me going to class.)
Here are some other examples of Mr. Van Eyck's work:
While none of these are works I would probably choose to hang in my own home, what always keeps me fascinated is the level of skill God gave people like this. It genuinely seems other-wordly.
I try to appreciate art in many forms and from all kinds of backgrounds, but when I see "art" made of glued-together pieces of trash (that look like nothing more than glued-together pieces of trash) -- these are the kinds of works I keep as a mental benchmark and it's really, really hard to see most "modern art" as actually being artistic.