I'd suggest as an artist, storyteller, imagineer and conceptualist he is/was a genius and more than that, he embraced and created a field of equally talented creatives that many filmmakers, fashion designers etc. use as a model today.
As a Director though he is without doubt flawed at every step and cannot therefore be considered one of the greats. To do so is to hugely misunderstand his skills, ability and input to the field of filmmaking.
Star Wars was (from all accounts I've read or heard) a nightmare to work on and mainly because of George, his demands and his Directorial style. There is no doubt though that the idea and final outcome is a treasure, but even that is flawed, hence why he was so desperate to rework the OT in the 1990s both due to the creation and development of new technology (in many ways under his own hand) and to unpick his frustrations from the initial shoots. I temper this comment of course by acknowledging that many (including myself) don't see these later 'enhancements' as actual improvements...
The Phantom Menace was IMO a fantastic conceptual piece by George and his team, it broadly makes sense, the characters should've been wonderful and exciting (and in many cases they really were) some of course found them silly and annoying, the worlds were realised as fantastical and in some cases, beautiful places 'before the dark times' but under George's directorial hand and in light of him having spent years re-polishing what he saw as his previous misgivings, his directorial frailties shone through again, he took too much time in the cut explaining every plot detail and explaining multiple backstories and forgot the simplicity of storytelling that actually he is THE master of.
Sadly the fan-base couldn't grasp the lack of lived in feel or come to terms with (what was an evolving art form) in CGI... something that along with most special effect techniques we see at the cinema to this day are only in existence because of George and his team of visionaries... it wasn't the film style we remembered, it is though most likely the film style George would've made in 1977 if the technology existed, which in itself is an interesting dichotomy.
So, George appears at this point to flip flop between the story he wanted to tell and the fan criticism/wants list and we ended up with the two worst Star Wars films in AOTC and ROTS, one a meaningless love story with a hint of Star Wars and the latter a fan letter to those that craved the dark side, but that left kids and fantasy/fun followers of the franchise behind.
More recently, and i'm sure after a great deal of soul searching he realised that the baby he had dreamt up and in turn delivered to the world was no longer something he could take any further. The fans had taken control in many ways and the franchise itself was at a point that only someone or something bigger and more imaginative could grow it further... step forward the greatest kids story delivery system... Disney Studios.
For all his directorial flaws and for all the issues, questions and problems that he brought upon himself with the special editions and later prequels, there is no doubting that George Lucas is a master dreamer, a storyteller extraordinaire and of course a ground breaker in the film industry when it comes to technique, technology and artistry... sadly what he lacked in later years were people around him to say no.
Take a bow that man for creating a monster, sharing it with the world and breaking all the rules of how to care for it!