Stay away from Simflyers sceneries, because they kill the framerates. Other than that, I recommend GAP3 Bundle (German Airports 3 Bundle, which includes HAM, TXL, DUS, LBC, BRE (I believe, not sure though) and MGL among others), GAP2 CGN and GAP2 HAJ. You could also buy GAP1 Bundle and GAP 4 Bundle as well.
For sceneries that are compatible with it, get AES and enough credits to activate the airports you have and can be used with AES. Trust me, AES truely enhances those scenery nicely.
Have got this pack and its excellent can highly recommend it. Also if you are an LAX fan look out for Cloud9's LAX, as its probably my favourite airport I can say that I was very much impressed by the product. They have a website at http://www.fscloud9.com/php/home.php?lang=EN. It would be great if they would make a patch for their MCO as a good Orlando scenery I have found hard to find. Would also love to know where to get a detailed KLAS there is a freeware one on avsim that is good, but would like one with moving Jetways and ground vehicles without upgrading to FSX.
Good deal for some great airports, just note that MIA and TPA are the two FT sceneries that yield the poorest performance. They look good (particularly MIA), but they don't perform as well as the latest FT sceneries. SFO and SAN are both awesome. 19.99 for those two alone is a steal.
Also fantastic. Gets a little less use due to the limited flight options (as opposed to FRA, SFO, etc.) but a really, really great scenery all the same. If you don't get it now, keep your eye on it for the future.
I'd also recommend looking at Flightscnery's PDX and Fly Tampa's SEA. Obviously, I use them a lot, but they're both among the best payware sceneries produced for FS9.