Which should I buy?

Darkest.

Better question is Darkest Hour or HOI3.
(don't know the answer to that!)

In my case any of the HoI2-Generation, because I want to play on my low performance notebook. But I am also curious, whether darkes hour or hoi3 is better.

and Arsenal of Democracy??

Not available on GoG.

I like the 2D interface way more than Hearts of Iron 3's 3D one.
Maybe HoI 4 will change my mind though...

Not yet...

I don't see the point in HoI2 being sold anymore, because from what I can tell you get it with DH, at least the version I have on Steam. In the launch menu you can choose DH with all its improvements; original HoI2 with the expansions; or some ''DH Lite'' version with only some of the DH features. Not sure what HoI2 Complete has that DH doesn't, really.

As for DH vs. HoI3... I haven't got much experience with HoI3 but it seems to be a bit more fleshed-out and complex in some ways. However, its manpower system is simplistic compared to how DH does it, and there are a lot fewer events, which can be good or bad. But I think what would make me choose DH if I had to is the excellent mods for it. Kaiserreich and FODD have given me many hours of enjoyment. HoI3 falls far shorter in mod variety. It also has a smaller scope, starting later and ending sooner than DH. I will say the espionage system is much superior than the clunky one of HoI2, and the DLCs have added a bunch of cool features like Lend-Lease.

Whenever I try to take up HOI3 I get stuck trying to organize my troops. I become bored and angry and quit playing.

Darkest hour definitely. Not only is it the more interesting game in my opinion but in the DH launcher you get the option to either load DH as a mod or not. If you don't it launches as standard HoI 2 Doomsday. So if you get DH you get both. As for HoI 3 I just couldn't get into it, both the mechanics and interface gave me headaches.

Darkest Hour.