The familial connection is oddly why I think Ultron comes second to Kang. Ultron is a villain of the characters you listed and an Avengers team lacking in those characters Ultron lacks any real connection with. Kang on the other hand wants to conquer our time and sees the Avengers as the challenge that needs to be conquered.
So the way I see it is Ultron is the greatest villain of certain Avengers while Kang is the greatest villain of the Avengers as a team.
Loki gets points for being the first.