Depends on your vehicle and your preferences.

I have a Honda Element and a hitch-mounted Sportsworks Transport T1 - the precursor to the Thule hitch-mounted racks (they bought Sportsworks a few years ago). Works great for me. Bikes are a breeze to put on and off and as has been noted, the entire rack comes off the hitch receiver by removing a single bolt (you can get a locking bolt setup).

I frequently remove my bike rack from the Element to put it on the back of my teardrop trailer. And as you can see in the photo below, with the bikes off I can still use the tailgate.

With one bike on the most rear location of the rack, I can open the Element's rear hatch. With two, no.

You also have the option of a swing-away hitch mounted rack. That may be a very good choice for an SUV or minivan in which the entire rear is a single hatch.

A friend destroyed a $5,000 road bike that was on top of his SUV by forgetting it was up there. He now drives a SAAB station wagon and the roof rack works very well on it.

Lots of great options. The best ones are not cheap.