I'm going to give you guys my pick, but I'll tell you right now, a lot of you won't like what I have to say. For the record, "least essential" to me means, pointless, stupid, and generally not thought out. I realize I'm going to catch a lot of hate for what I'm about to say, but:
Qui-Gon Jinn.
Yes, Qui-Gon. How could anyone hate Qui-Gon, you ask? Conventional wisdom tells us that he was one of the few bright spots in the prequel trilogy because... well, because he was Liam Neeson or something. But when you really look at it, Qui-Gon Jinn was one of the most pointless characters ever.
The entire purpose of Qui-Gon was to find Anakin and then die. Literally, that's it. Basically, George Lucas created the character and then realized he had to get him out of the way as soon as possible for continuity reasons. The story as we all know is that Qui-Gon is a maverick Jedi Master who finds Anakin, the possible Chosen One, and really wants to train him. Meanwhile, his real Apprentice Obi-Wan is a more level-headed dude who is more or less pushed to the side the entire time, and ought to be rightfully kind of insulted by the fact that his Master is trying to push him through the ranks in a haphazard and unceremonial manner just so he has room to take this kid as his new Apprentice, because the rules say you can only have one Apprentice. "Yeah listen guys, Obi-Wan is ready to face the trials and be Knighted or whatever, now let me train Anakin already." That's pretty much what Qui-Gon tells the Jedi Council, without much concern for what the Council itself might have to say on the matter, because when they disagree, he more or less gives them the finger and tells them he's going to train Anakin anyway.
But then Qui-Gon dies, paving the way for an inexperienced Obi-Wan to train Anakin. But why the sudden change of heart? Qui-Gon dies, and just because of that Obi-Wan is now hellbent on training this little brat, who he seemed thoroughly irritated with the entire movie, even going so far as to call him a pathetic life-form, as well as a potentially dangerous threat? I don't buy it.
It had to happen of course, because the original movies clearly establish that it was Obi-Wan who trained Anakin. So it really begs the question, why go to all the trouble in the first place introducing a character that needs to be swept away almost as soon as he is introduced to keep with continuity? It's not like there is anything that makes Qui-Gon particularly unique, except maybe the fact that he's willing to tell his direct superiors to go eat a can of worms whenever it suits him (but Obi-Wan does the same thing too, and later so does Anakin). Qui-Gon Jinn should have never existed at all. Instead it should have simply been Obi-Wan Kenobi from the start. Obi-Wan should have been the brash, idealistic and naïve young Jedi Knight who discovers Anakin and really wants to train him despite the advice of the Council. It would have provided a nice contrast to the wise, sagely old Obi-Wan we see in the originals, giving him room for some nice character growth throughout the rest of the prequels. And it would have set the groundwork for a real friendship between Obi-Wan and Anakin, which was also something the originals established, yet the prequels failed to convincingly reproduce. There was simply no need for Qui-Gon at all. Obi-Wan could have filled the entire role himself, and with better results for the series as a whole.