(This is NOT hardest for speedruns... just plain beating it)
Perfect Agent:
(Obviously its just my n00b opinion)
Oh, and including bonusl levels, I'll just include both.
Not in cluding bonus levels:
I had a lot of trouble with Villa, it seemed like there was always a guard from somwhere that was hitting me, and I was also low on amo.
For me, the easist level was Area 51 - Escape. I didn't have any problems with it. I found it easy to kill of guards with the SuperDragon.
Including Bonus Levels:
The hardest in the whole game to beat on PA for me is Maian SOS, it is very hard beacuse of: You start out with low health, amo is a huge problem...and there are lots of guard... Also when it seems like you've cleared out all of the guards, theres like a respawn thing, more and more guards keep coming from that.
NOTE: I would say WAR! is a close third to Villa.
(Easist is the same)