First of all, my first thread here in VGchartz. So be gentle :)

The Xbox 360 version of "Call of Duty 4: Modern Warfare" has a rating on Gamerankings.com of 95/100 whereas the PS3 version is 94,2/100 and the PC version is "only" 91,9/100.

So I wonder, what makes the game better on Xbox 360?

It looks probably the same and the online functions should as user friendly on the PC as on the Xbox 360. And as FPS the PC ought to be the first choice for many.

What are your thoughts?  Do you know another game that has been rated differently on which console it's on?

The only thing i ever think of when i hear 360 games having higher scores than their ps3 counterpart is the addition of achievements .. i know it may seem silly to others, but if i had both consoles, i would almost always choose to get the 360 game just for that reason

that doesn't explain the drop off of the PC game compared to the other two though

Nothing makes it better on the 360, from what I've seen. If you look at how the gamerankings average is calculated, you'll notice that different people have reviewed different versions. The 360 has a 97 from TeamXBox and a 100 from Official XBox Magazine, while the PS3 doesn't have a score from a platform-specific source.

The PC version got an 85 and an 89 from PC Zone UK and PC Gamer UK, respectively.

Shooters get lower scores on the PC because PC fans expect more from Shooters than console fans, it has been this way for a while now. The games are probably identical with the PC having the edge in graphics on machines that can handle it but PC gamers are a bit spoiled in this genre.

The difference is so small that I don't even see why anyone would care.

It's not really better on 360, it's just from different sources and graded on a curve.

All three versions are essentially the same.
If you have a really good PC, get the PC version.
If you're not already paying for Xbox Live, get the PS3 version.
If you like achievements, get the 360 version.

Some of it could be explained by different reviewers. For example the official PS magazine would only review the PS3 version and the official 360 magazine would only review the Xbox 360 version. If reviewers who only review one version give different scores then that will reflect in a different overall average.

If you want to see whether one version is better than another however, just look at the places that review the game on all formats. If the scores are different then then you could see whether one is better than the other.