Now, putting aside that the concept of "greatest" is debatable.
The greatest ever Star Wars character...
... from the Original Trilogy?
... from the Prequel Trilogy?
... from the old Expanded Universe?
... from the new Canon?
Luke, Han, or Leia? That's a hard choice. The young guy coming to greatness, something very relatable? the hardened older guy that still has a heart of gold? Or the woman that defied standards and role models showing the balls?! You tell me I'm forgetting Darth Vader, with "its magic", the Emperor, heart of all evil, or Tarkin, a true villain using fear to keep local systems in line? What about the See-Threepio, human-cyborg relations fluent in over six million forms of communication, and his counterpart, Artoo-Detoo? It's not a choice I can do... they are all too great.
In the Prequel Trilogy, it would be a bit easier as I would probably focus on Obi-Wan Kenobi, I know he appears in the original trilogy as well but I already spoke too many names and it's in the prequel he said "Hello there.", and on Senator Palpatine cause he worked so finely leading to the rise of the first Galactic Empire, for a safe and secure society.
Then, the old Expanded Universe had... a lot of characters worth of attention, from Ysanne Isard to Thrawn, to Wedge Antilles and the pilots of Rogue Squadron, to the children of Han, Leia and Luke, further forward to Darth Krayt or further backwards to Darth Bane or Revan.
In the new Canon, this is the way, so the Mandalorian? The new Thrawn? Omega and the Bad Batch? Rex? Rey? One of the Jedi from the High Republic? Ashoka Tano? I'm not sure honestly.
Okay, okay, you're asking me one name... only one...
Obi-Wan Kenobi.
The man. The legend.
Nothing else is needed to be said. He is Obi-Wan Kenobi, of course he knows that name, that's him.