>>1790359meme mode disengaged
The IL-2 series is pretty much it for WW2 stuff. Cliffs of Dover is available for a low price (£5 or so on sale) and while horribly buggy unmodded, it has an extensive community patch available directly through steam that makes it into a really solid sim, worth far more than the price of entry. Said patch also has free content (maps, aircraft, flight model improvements) on the way. It focuses on Battle of Britain stuff, not surprisingly.
Battle of Stalingrad is supposedly also very good, and still supported by the original devs, but it's more pricey. Focuses on the Eastern Front.
Rise of Flight is a bit of an oddball, being a first world war flight sim, free to start with two aircraft, but additional aircraft have to be bought individually or in packs. Despite this it's a super solid sim, with a damage model second to none in my opinion.
There's also DCS which I know little about, but it's supposed to be extremely detailed, with a few free aircraft to begin with, and a mixture of WW2 and modern stuff availible as DLC, also has helicopters.
