I really like rendering of http://beta.map1.eu
E.g. on zoom level 5 in Europe: In OSM render I have no idea where are the state borders.
Or compare Paris on zoom 8: Can you differ higways, main routes, trains and rivers?
Compare both cases to map1.eu. I think it beats in readability even Google maps, which is heavily focuses on roads (and e.g. rivers are hard to recognize).