A2A 377 is a regular. Lots of fun once you actually read the manual. Even if you don't buy Accusim it is a blast to fly.
Ted
That's the best choice, especially with Accusim.
However, that also depends on the level of complexity you want, Falcon.
The A2A 377 is not going to start with CTRL+E, and once all the engines are started, you're not going to fly without caring about your engine parameters.
This is why, if you are looking for a good payware, I'd rather recommend you the A2A B-17 with Accusim. It's even more complex than the 377, but the major difference is that you can assign the engine handling to the crew, so you can actually fly without watching all the time your engines.
Now, in freeware, I would tend to think about the Constellation from Manfred Jahn, very complex as well, but it's a FS9 port-over.
There also of course the defaut DC-3, and it's variants called Awesome4some.
The Electra made by FS-KBT is native as well, but the VC is quite simple...
The Convair 240 made by Brett Henderson (hope I wrote it correctly :/ ) is FSX native too, and it has a virtual cockpit that is a remodelling of the default DC-3 (new intruments, and different windows I believe).
The Howard 500 from Milton Shuppe has been converted to FSX native as well, you'll find it at FSNorthwest.
What else?....