When a game's marketing material needs to indicate that it's for console or mobile, it's an easy process. Each platform has a single basic logo that is instantly recognisable and crystal clear in its intentions. Sure, there's some variation here and there (eg. displaying the PS4 logo without its PS icon, or using Android's green robot instead of the Google Play button), but those variations tend to add flexibility rather than complication.
When you want to slap on a logo on your game poster, box, or trailer indicating that your game will be out on PC, you have about as many options as the other platforms combined....and most of those options have problems.
Then there are "Games for Windows" type logos, which make sense - but those are reminiscent of the Games For Windows Live service, and the last thing you want to do is traumatise potential customers.
Many devs simply use the Steam logo, since that's virtually synonymous with PC games for many people. This certainly makes sense in some situations, but has problems in others. Obviously, it excludes other storefronts like GOG (or perhaps a Humble Widget on your game's own website). It's also problematic for games in development which haven't gone through the Greenlight process yet. Stating that your game will be on a platform that hasn't officially accepted it yet is basically false advertising, no matter how unlikely it is that your game will fail to be Greenlit (going by the average quality of Steam New Releases nowadays, it's pretty unlikely).
Some more broad and generic PC game logos tend to carry a "CD rom" or "DVD rom" tag, which is logical (though still unnecessary) on a physical boxed copy of your game, but makes little sense anywhere else.
The logo on the bottom right is about as close as we've come to a logo that is both generic and widely recognised. This is certainly an adequate option in terms of clear messaging. But it's pretty ugly. Those clunky, octagonal letters crammed into a too-small box look like something designed by a programmer from the 386 era, rather than a contemporary graphic design. To my eyes, this logo placed next to a slick and modern logo like the Xbox One logo would send the message that the PC version is archaic and technically inferior, when the opposite is likely to be true. But I'm a PC Master Race kind of guy, so maybe that's just me being too precious.
The best PC logo I've seen comes from Fallout 4. There's nothing magical about it - it's a very simple typographical logo featuring a square, a line, and a fairly generic font. But there's an elegance to it. It looks strong and clean. Unlike the 'octagonal' one above, it breathes, and its negative space forms pleasant, nicely balanced shapes, not ugly and jarring ones.
So I've recreated a high res version of the logo, with a few minor tweaks for clarity. Here it is on a poster of my game Spryke.
But wait, isn't this infringing on Bethesda's copyright?
Uh, I guess you could say that technically it is. And if Bethesda ask me to take these down, I will. But honestly I'd be shocked, and a little dismayed, if that happened. The logo has nothing to do with any of their IP, and other people using it on other games would in no way constitute bad faith towards Bethesda. I can't see how it would benefit them in any way to not want others to use it, and - though I might be wrong - I don't believe they would care.