I've got a Thule tow ball mounted two bike rack. As far as I can remember it is a euroclassic something or other, can't remember the model number and it appears from their website that they have updated their range since I bought mine. Anyway the point is that as you said that one of your requirements is that it should be ok left in place then I'd say that one of those will do what you want. I leave mine on my car permanently, all year round, without any ill effect. They are very well made and mounting and dismounting my bike takes seconds. The tilt mechnism for accessing the tailgate works ok but it is a faff to do, on mine you have to partially release the ball mount clamp and screw out a stop mechanism to enable it to tilt down, as I say it works ok but it is a five minute job to get in the back of the car and do it all back up again. If you would need to do that often it may become a pain or you may get practised at it and get it done quickly and without hassle. Since I rarely use the tilt function it doesn't bother me and the ease of mounting and dismounting my bike is what I like about them, that and the facts that it is very well made and the bike is securely held when mounted.
One thing to bear in mind if you do want a tow ball mounted rack is to make sure it sits on the tow ball squarely. When I bough mine I was told that they do not fit properly on some tow ball designs. I have a swan neck tow ball and it is ok, but my wife's car had a different design and while the rack would mount on it, it would not sit level and square. I don't know if this applies to other makes but it did to the Thule one I chose. Maybe the newer designs do not suffer from this problem, my rack is about 5 years old now, but something to ask about if you consider one.
Anyway, I am very pleased with mine.