I was in the Van Gogh Museum in Amsterdam in February and stood for about 30 mins just staring at his "Wheat Field with Crows". The whole place was full of greatness. Next day I went the The Hague to see Varmeer's "Girl with a Pearl Earring". Sublime.

Coming up to date, I like video artists such as Ori Gerst (see here) and just about anything by Wolfgang Tillmans. Ranger1 mentioned Turner: one of the all time greats, but we owe it all to Giotto.