I’m posting here, because honestly I have more faith in Flavien’s opinion than most other self proclaimed “game programmers” (including some very well known game studios).
There are certainly quite a few to choose from. Here are a few:
Raknet
Enet
Lidgren
libyojimbo
or something completely different
I’m sure each has it’s pros and cons, but if I were interested in making an MMO Flight Sim using UDP… Which would you recommend and why?
Glenn Fiedler (libyojimbo author) recommended Raknet for MMO’s and libyojimbo for everything else. That being said there’s a reason we’re using neither 
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I don’t want to compromise security of Battlescape, so I’m not interested in your solution.
Can you provide the “reason” you’re not using them? I understand libyojimbo may not be ‘mature’.
Most of those libs do not make use of modern Windows features. IOCP would be the minimum when you speak of MMOs ( Raknet for instance does not support it AFAIK ), but there are many new features ( forgot their names ) designed to reduce the amount of data copies transiting through the TCP/UDP/user layers which you can’t have if you use an external lib.
Still I wouldn’t recommend writing your own networking lib. It could take a lot of effort and time and if you aren’t serious at commercializing your game I think it’d be a waste of time, which would be better spent in other systems of your game.
On Windows you’re thinking of Registered IO, NetworkDirect, and various NUMA features such as setting socket processor affinity
I LOVE you guys!
Answers like this are exactly why I asked this question on your forum. Thanks a ton!!!