+1
An interesting question, but it's kind of based entirely on a point of reference (and there's much debate over whether there are any actual points of reference or only those presented by a little more than a half century of media.)
So, based on the most common reports of encounters or abductions, probably the aliens at the end of Close Encounters would be convincing.
Based on the idea of life-forms that may be similar to different types of biology on Earth, but which followed different evolutionary paths, then I'd have to go with the aliens from the Alien franchise (or virtually any other popular aliens that seem somehow insectoid, crustacean, fish-like or reptilian).
If we buy the idea of "panspermia," then aliens could look very similar to or just like humans - so take your pick from any humanoid aliens from Star Trek or other sci-fi series that feature human-looking or humanoid aliens.
The aliens from the Predator series were a nice hybrid of humanoid & unknown other.
My guess is there are probably many different aliens that we might not even immediately identify as "alive," such as huge gas clouds, beings consisting of pure energy, microscopic or gigantic things that resemble bacteria, immobile beings that might look to us like rocks, or even living planets or stars. So on that note, a creature like that depicted in The Blob may be one of the most convincing looking aliens.