I recommend a Rhino for police missions.You can get them by getting 6 wanted stars.Go hide Somewhere like the Portland Gunshop so the police can't take a shot at you because the glass is blocking the shots.When you saw a Rhino going past, run out to get the soldier's attenion.Then the soldier will try to kill you.Now,Go into the rhino(NEEDED FULL ARMOR AND HEALTH OR YOU WILL DIE)Then, type the "Never Wanted" Cheat. Then You'll be on a Rhino!Don't worry you wont get caught as you have the "Never Wanted" Cheat

Good and bad things about Rhinos:

Rhinos can destroy cars that hits your rhino hard
Rhinos are slower than normal police car
Rhinos does not catch fire easily unless you hit it about 30-95 times.depends how hard you hit it.
Rhinos does not drive on water
Rhinos can kill passangers in about 1-4 hits

You can easily win more armor points with a rhino
There is no more ambulance trucks. only the normal ambulance truck.
You must be fast in ambulance mission as there are only 1 min to 3 min