There are so many makers, and so many threads, that really some reading at the top of this section would be a better place to start and then come back with specific questions.
None of us are lawyers, so you should consult your local laws/police/gunshops instead a bunch of gun nuts on the interwebz when we are talking about federal jail time.
Reliable is a great place to start. That should be with every gun. What do you want the gun to do? Target shooting? Varmint hunting? Zombie defense? How many rounds do you think you will shoot in a year? Do you shoot every weekend now, or just occassionally?
All these questions will help us help you.
Or you can go with the blanket statement of "get a Colt/Noveske/BCM/YHM/DPMS/RRA and don't worry about it".