The National Gallery

I didn't have a lot of time going through here and it felt like I was disrepecting something sancrosanct by passing through the Halls of the Masters without always giving them their due. I loved da Vinci's work, the Dutch & Flemish painters; getting to see paintings that I'd studied at uni.
Their entire collection can also be viewed online at:
http://www.nationalgallery.org.uk/.