His acting ability in the Trilogy and many other quality films he often lifts the scenes he is in, or the film itself - where conveying emotions, strength or resonance of character, or bringing innovation to his craft he was a step or 3 ahead of many actors around him.
Except in ROTJ, where he appears to convey no emotion at all and could only be bothered to show up if he was allowed to sit through all of his dialogue. Actually if you compare his sitting exposition scene in Star Wars to his sitting exposition scene in Jedi, it’s a great example of the gravitas that great acting can lend to otherwise mundane exposition contrasted with mundane exposition that’s just being phoned in.
His performance in Star Wars is remarkable though. He almost does make you believe in the Force. He definitely tempers Mark Hammil’s performance, which would probably be a little grating without the wizened Obi-Wan as a foil of sorts. He kind of sells that movie for me.