You run into some thorny problems with how you define complexity, too.
I ran two 4e games for most of its lifespan. I found it easy to run - vastly much more so than 3e! However, as
@GreyLord said, player complexity was much higher. For my two groups in particular, very few of the players were strongly invested in the mechanics of 4e, which meant that I ended up having to spend several hours helping them every time they levelled up, providing advice and resources. That made levelling up such a slog that I used to dread it.
But the thing is, I believe that a group that was invested would likely not even recognise the problem I faced; it seems to me that a lot of 4e's complexity was in the choices and a limited set of interactions. It's a complexity that's easy to learn.
Basically, I'm saying that complexity means different things to different people in different situations. Sometimes complexity can be hidden by good design (the board game Scythe is an excellent example here; it can easily take 30-40 minutes just to understand the rules... but it's common for players to get to the point within a single game where the average turn is about 5-10 seconds), or sometimes poor design can make something seem much more complex (yes, OD&D. I'm talking about you).
I personally define complexity as anything that acts as a barrier to understanding the rules. By this terrible and arbitrary measure, I'm going to order the editions like this:
Starting from least complexity, and limiting to things I've actually played:
5e. It's easy to understand, the basic rules are free online (so easy to obtain), and the SRD contains full chargen instructions
3e. A relatively unified system. SRD is available and has a lot of information on how to play, if not actual mechanics of chargen (although Pathfinder did include some...)
4e. Simpler as a system than 3e and (perhaps) 5e, but with more barriers to entry. You had to pay something or know someone who had.
2e. I always found it a bit easier to understand than 1e, although I never had the chance to play 1e, so maybe that's just bias.