If you are here's a great start.
OFPEC Mission Beta Testing Some reallly good ones come thru there. The guys really appreciate any input they can get.
Right now I am struggling with a really cool "Lone Wolf" mission by LCD.
LCD's Guardian Angel If you'd like to try download it and give him some feedback. Oh...be forewarned he speaks LCD"lish, which is kinda of a Hip Hop Minglish.
I try to play and test a couple of em whenever I have the chance. And after you have tested a few, you can put your own up for Beta Testing.
FYI