The best looking source "game" i have played is FakeFactorys Half Life mod. Yes its a mod and not a "game" per se, but this is by far the "best looking" implementation of source I have seen so far.
Because this is an ambitious mod to say the least, you need a really good pc to do it justice. Then again, the same can also be said for Crysis for that matter.
Having said that, its interesting to note what the author of the mod wrote in the readme file for the mod itself. Heres a snip:-
"PERFORMANCE-WARNING:
The Orangebox/Source Engine is getting outdated and performs poor compared to newer engines like the Cry-Engine or the Unreal-Engine.
It's almost impossible to get good framerates with enhanced viewrange, hires-textures and high polycount. Therefore prepare yourself for fps below 40 in some open areas, unless you're using the latest NASA supercomputer.
Currently i have reached the source-engine limits in almost all areas. Unfortunately the limits are quite low. The current build of the source engine can't render detailed open areas or nice foliage without hitting this limits. (Compared to the Cry-Engine: Source engines 100 % entity limit equals approx. 5 % of the Cry-Engine limit)
I'll take a break now, hoping the next engine version will give the possibilities, i need."
Having said that, the results are nothing short of stunning. Heres an ingame screenshot from the mod.
If you're interested, then have a look at this link which shows a -direct comparison- between screenies from the hl2 default game and the FakeFactory mod version.
Half-Life 2 vs. Cinematic Mod 10: Visual comparison - Update: New darker look - Cinematic Mod 10, Half Life 2, Fakefactory, comparison - PC Games Hardware
The "mod" in question is over 10GB in size, which is large and rivals many commercial stand alone games. It also requires hl2, ep1 and ep2 to be pre-installed and "run" at least once before you can play this mod.
IMO, this mod highlights the current limit of the "source" engine as it stands today.
Is the mod worth the download? IMO, if you have a good PC and like HL2, I certainly think it is. Decide for yourself....