Well, let's take a look at all the random factors in the game. Levels with annoying NPCs are Extraction, Escape, AF1, Crash Site, Deep Sea, Attack Ship and MBR. Extraction is probably the friendliest in this regard whereas Attack Ship and MBR are the worst.
Defection - the glitch is only random if you lack the skill. On PA you have the programmer to deal with if you take the glitch strategy, or the ground floor guards if you do it old-school...
Investigation - Invest A is almost all luck, we know this well. PA is pure skill and perseverance, however.
Extraction - Dr. Carrol is the only properly random element here, the rest is fiddly strafing...
Villa - Probably the best skill level. No NPCs, no timed events that you need to wait for or time your objectives to coincide with...
Chicago - Still almost entirely skill, just way shorter than Villa so it gets ranked lower.
G5 - Skill again but with the annoying guard patterns and horrible fiddly strafe patterns. About as difficult as Villa but tests different skills.
Ruins - Because of the annoying pillars you end up PR'ing about half as fast as on any other level here... but ignoring that it's all strafing skill.
War - all strafing again; shorter than Ruins or Villa, and with a ridiculous number of sticking doors...