The Black Project Drone. That's for the look, a bit less for the practicality (the link between the sphere and the nose can be easily broken). In terms of practicality and of resistance to injuries, I would then choose the Police Drone.
I like the military drone the best. It's worth noting that you've done a great job with each of these in conveying the purpose of the design. For example, the police drone really looks like something a civilian police force would use, rather than the military.
The Black drone has the best design choice, and I’d be interested to see it developed further (Love the sphere). However I also feel that it has the weakest base of the three. It lacks the solid shapes of the other two, and while I can see the skeleton/Gothic ideas behind it, I think It needs to be pushed further in that direction. Also, I feel uncomfortable with the size of the neck between the forward cockpit and the big sphere. It feels really weak and makes me think it's fragile.
The Military drone has an interesting contrast in angles, I like the bulbous nature of the forward head and the way it contrasts with the angular sections in the back. This one is probably the strongest base, and something I can easily envision its identity as a drone and the textures along the bottom are very attractive.
The Police drone is my least favorite. The line from the cockpit to the back is too strong in my opinion, and makes me think of both a whale, and [link]. And while I recognize your styling it after VTOL vehicles, I think It looses it's identity as a drone to the shape.
I like the way you kept just a tad of "family resemblance" but yet kept them individually interesting. I'm not often given to Star Wars references, but for contrast there's more difference than found in the Tie Fighter/Advanced/Interceptor family yet you can still tell they are related.
I like the rear thruster on the police drone but the black drone is legit.. probably really easy to model though. If you want a challenge i think the military drone would be the most fun. Plus you couldn't show anyone the black project drone.... since it's black project...