Research experiences are often hard to judge from the outside - e.g., did someone really make original contributions to research, or did they clean glassware for a few weeks? 1st authorships put much of that ambiguity aside. You really have to work for those, and the work has to pass the peer review process. For that reason it makes whatever research experience you have shine. If the residency you're applying to couldn't care less about research, it's little better than a talking point, however.
1st author is exponentially better than anything else. It probably depends on the project and how duties are divided up, but in my experience as 1st author I did 95% of the work. Other authors were there to help me when I needed advice or opinions, but I had to implement everything we decided on. And at least one author who was 4th or 5th I had to put on at the behest of one of the co-investigators - I literally only met the guy once and never heard from him again. I think he might have been applying to med school at the time.