In reply to ckosacranoid :
Follow my guidance...
-For "beat the time" missions, beat the race then call in your 'bob BEFORE you get in the car you won. Fly back, hook up, fly up as high as you can then pitch forward straight at the Vehicle Warehouse (VWH).
-For "security guard has a key" missions, stealth is the main objective. Land the 'bob nearby, stealthily pop the guards with silenced weapons and make sure they are not in sight of another guard or person. Get the key, drive the car out, pick it up and fly away. Even if you do catch heat you can usually get away from them if you focus on flying low and fast.
-For "crime scene" missions, pull up your phone and call Lester before entering the crime scene (either on foot on in the 'bob). As soon as your wanted level pops up, select "remove wanted level" and you will have no problems.
-For "DUI" missions, same concept. Have Lester on the line as you hook the car.
-"Car chase" missions are tricky. You need to land close enough that you can see the car, without triggering the chase. Snipe the driver, get the car, THEN remove your wanted level. Otherwise you will get a 3-star wanted level both when you shoot the driver AND when you collect the car.
-"Movie stunt", "photo shoot", "street race" and "not abandoned car" missions CAN be done stealthily and not attract attention, but it's hard. Your best bet is to snipe the guards/ gang members from afar, then move in and scoop up the car with the 'bob. Then make a bee-line for the VWH.
-"Car bomb" missions can be completely avoided if you grab the car with the 'bob and never get in it.
-"Race track" missions are as simple as blocking the exit, sniping the driver, and taking the car.
-"Offshore" missions are tough, but my method is proven to work. Land 'bob somewhere nearby and call in your CEO Buzzard if you have one. Fly over the boat and using homing rockets kill all the guys on the boat. If you don't have a Buzzard you'll have to snipe them. Now, grab 'bob and hook the car. Fly STRAIGHT UP as high as you can until the buzzards they send after you are directly beneath you. Then bee-line it straight to the VWH.
Any missions I've missed?