Since graphic-engines evolve as technology gets better for every 18 months, it makes it a bit difficult to judge the graphics objectively since there's always a better looking game around the corner (right now, it's Crysis). Usually, one can just judge a games graphics on the art design and what they were able to do with the engine at the time.
That being said, Unreal 1 had the best graphics of its time (1998). When playing with a Voodoo 2 3D-card, nothing came close to it, no Quake 2 and certainly no Half-Life.