The gag about aliens confusing cars as the dominate lifeform goes at least as far back as the Hitchhiker’s Guide to the Galaxy. The Ford Prefect was a car only sold in the UK and not in America making the joke easy to miss (also a lot of people read it as Perfect not Prefect).
Dogs. L. Ron Hubbard said so. 
But, seriously, it would have to be humans. It wouldn’t take much observation to see that humans run things; you could pick that up as soon as you decoded TV broadcasts.
Aliens would probably be able to see the reptillians for what they are.
So, the reptillians.
You are on the right track in trying to define “dominant” but bacteria beat us there on all counts. Anywhere we exist they exist, within and on us, we us serving them their food and often taking care of their waste. And they exist in many places we cannot and no other organisms on this planet can. Forget about population numbers, by mass we are beat out even by termites and krill, but bacteria beat us out by several orders of magnitude. They were here long before us and will be around when we are gone.
Not sure how we can even delude ourselves into thinking we are “dominant.”
Of course in reality any alien will be alien-centric: which entity seems to them to be most like them?
The thing with the bacteria argument is they can relate to humans as commensals, mutuals, or as parasites. We’ve pretty much handled the parasitic aspect (or we wouldn’t have grown this numerous.) Bacteria and viruses will win only if they can keep the human population in check. Well, they haven’t since 1919.
The best argument for bacteria is their role in the nitrogen cycle, to which nearly every organism is dependent.
Huh?
They win more the more we multiply. Everyone of us is many many more of them and even in cell count at least as much of them as us. We are the multi-generational ships they will colonize new planets with and once there they will spread into more niches than we do. Hell even if we don’t do ourselves we may serve to spread them on the structures we create for their sake.
I first heard of cars being the dominant species on Earth in a Michael Crichton novel. Don’t recall which one right off hand.
Smart phones.
I suppose, for that matter, they could have super-human eyesight, or be observing in something other than the electromagnetic spectrum. Maybe they can observe humans on earth from the edge of the solar system. Just takes some pretty sensitive sensory organs.
The alien observing the Earth will consider itself to be the dominant species.
Well, sure, if they’re perverts.