here is a short list of characters and other things i beleive could resonably defeat or kill a master Jedi in battle, but unlike the other guy's list, I'm posting WHY :O
1) Kirby (the kirby series)
While Jedi can easily block physical objects, as far as i know they are not resistant to wind and have no effect on it, thus can be inhaled and swallowed (hello Jedi Kirby), force crush and any amount of bodily disfigurement will not work on kirby, this negates the "hold him up in the air and crush him" method (also note that like someone earlier said, it has never been known for a jedi of any power to use force and fight at the same time, so no 'hold in the air and stab' either) only the lightsaber and possibly Force Lightning could actually cause harm to him (as far as i know, excuse me if i am not a walking dictionary of force powers). while kirby admittedly wont win hands down every time, he would have many more victories than losses.
2) Doctor Who (doctor who)
Reasons have already been explained by others
3) Sonic, maybe (the sonic series)
I'm probably falling into the trap of 'i like him so he'll win' but regardless of precognition its nearly impossible to react to something that can travel at the speed of sound, and with chaos emeralds, the speed of light, with super emeralds(play sonic 3&knuckles), exceeding it, with the master emerald. limitless power, oh well. (also note, he's absolutely invincible as super sonic and beyond, despite what happens in the games when you're crushed; that was just so you can't run around inside the walls)
4) Replicators (stargate)
think about the name. also, the just keep coming back from the 'dead', force lightning is ineffective, force push, blasters, physical dismemberment and disfigurement, mind tricks, telekinesis(can only be done on one or two things at a time, sorry =/) all do not work. With all of his tricks useless, he's doomed from the start. Although i'm not sure if this is cheating in the 1 vs 1 department.
5) Xavier (xmen)
Smarter than the jedi, and does not have to do or dodge anything if he can get in their heads and make them kill themselves.
6) Phoenix/Dark Phoenix (xmen)
de-atomizes everything. win.
7) Lyta Alexander (Babylon 5, notice i capitalized that one)
Massive telikinetic and telepathic powers, can read the jedi's every move, every intended move and react long beforehand, can make them kill themselves, can cause their brains to implode, can close up arteries and airways inside the jedi, and an assortment of other very nasty ways of killing are at her disposal.
8) a Level Designer (life)
just a little joke i thought of, obviously would only work on the game ones. just use your imagination =)
9) Ganondorf (Legend of Zelda series)
holds the triforce of power, which means he holds more power than anyone or anything else that exists, and force is a power. Can't be hurt by anything other than light arrows, the master sword, and his own projectiles. cannot be killed, at most sealed.
10) any God of Death (most books, games, films and anime have one.)
obvious.
11) Ness, Lucas, etc. anyone PSI powered (Mother series)
Jedi can't stop other people's telekinesis, and Ness and Lucas are better at it than them.
12) the last boss from Ikaruga (ikaruga)
If you don't know what it is go find out(google video will probably help) imagine trying to beat that, on foot, while susceptible to both white and black bullets at all times. Also, with no means of killing it...
13) Pokey/Porky (mother series)
He has a time machine, he'd stop the Jedi from ever having existed (dunno why he never did that in the mother games, but oh well)
14) Master Hand and Crazy Hand (kirby and smash bros. series')
Have you ever noticed they never actually die? If one gets weak the other gets stronger(because they are representations of opposite things) the worst that can happen is what Tabuu did(no spoiler yay). (I'm not putting Tabuu in, so far as i know he can die, pretty easily at that)
15) Ultimecia (FF VII)
Came close to fusing all of time together, you can't have that amount of power and lose to a silly little master Jedi(yes she lost to squall, but they couldn't exactly make an impossible game could they?).
16) Super Paper Mario (super paper mario)
He just flips them into 2D, which they are completely unused to, upon which time he has a ridiculous advantage. (I'm fairly sure the Jedi will be slightly distracted by this too

)
17) The Flood (halo series)
It starts as just one, but even if it dies the spores will spread...
As stated in the games, the only way to kill them is to kill everything of sufficient biomass to sustain them (starve them by killing all possible food sources everywhere, in other words), which would mean he'd have to kill himself and everyone else in the galaxy. Flood win every time no matter what happens.
18) Link (Legend of Zelda series)
also known as the Hero of Time. yeah, I'm beginning to see a slight weakness for Jedi here, although i suppose it isn't exclusive to Jedi.
19) the Bridge of Death (monty python and the holy grail)
If you've seen it, you'll get me. If not type 'monty python bridge of death' into google videos, or something similar. From what I've seen it will win 1 in every 2 'fights'
hmm, 20 seems like a nice round number to finish on, okay, gonna have to think of a really really good one...
20) Time itself (everything)
Time beats everything, every time, no matter the circumstances, no person, thing, or any other possibility of something that exists can withstand or defeat it. EVERYTHING ends with time.