I personally find Milkshape to be one of the easiest programs to learn. Been doing modelling and whatnot for a few years now, and it's really the only program which I've really stuck with over time.
I primarily choose Milkshape for character and weapon modelling however. I feel it handles this well. So far as architectural modelling goes, I've recently fallen in love with google sketchup, its super simple to learn and produces excellent results in minimal time. (There's a bit extra you'll need to perform however if you want to make your sketchup models FPSC ready.)
Programs I prefer not to use because of what seem (to at least me) too complicated: Blender, and 3ds Max. Keep in mind this is only my opinion, and I'm sure there are alot of users who utilize these programs with great results. I just personally remember the feeling of intimidation I had the first time I opened up Blender!
Hope this guides you in the right direction, good luck!
I feel like you're eyeballin' me, dawg!