OP - You haven't told us your budget. Needless to say, that makes a huge difference. Car service is the high end because you will be met at baggage claim, driven to your destination without stops and dropped at your destination. Public transportation is the low end and involves multiple changes and long waits. Super Shuttle is one up but can mean a 2-1/2 hour trip for something which should take 1 hour while you wander around dropping off and picking up.

IAD offers all of these, so you've got to make a choice based on budget.