Christopher Reeve was the best, most believable, Superman. Only an actor like him could really pull off that duality, at least no one else has so far. It's to bad the film is so dated with it FX because kids these days will never get that, it's hard to describe, that super feeling you got when Superman did his reveal in that film (Can't find a good clip of this on youtube but the one below shows what DM & I mean about his acting)).Frankly, it wasn't until Christopher Reeve brought the role to life on the big screen that anyone really started taking the "Clark Kent, Mild-Mannered Report" disguise seriously. The difference between Clark and Superman is as different as night and day, and was made very clear in one scene in the first movie where he's about to reveal himself, and you can literally see the change between the two identities just from the way Reeves changes his posture and tone of voice.
The moment I read the earlier post I wanted to find and post this exact same clip! It's a magnificent little bit of acting. It actually makes the Clark Kent / Superman thing believable! Christoper Reeve was great and I can only appreciate it now as an adult how good he was. (Though I recall enjoying it plenty as a child).