Executes are not strictly burst damage since they require more damage beforehand to do their own damage. Pantheon I also consider more of an assassin, not a fighter because to one-combo people you really need all the AD you can get.
For Garen, sure, DMP and his Q empowered auto is a decent chunk of damage, and his ult can drop you from around 1/3 HP if you're squishy/the villain, but overall his damage isn't really bursty, his E is a huge portion of his damage, and only if he can spin on you the entire duration, and only if he's hitting only you does it do really good damage.
Irelia, I have never actually seen her burst anybody without building glass cannon, her trades are fast sure, but they aren't just a ton of damage in a split second, it's decent damage over like 2 seconds for trading, her all-ins give her good sustain and extra damage over the duration of Hiten Style, and her ult can do OK damage across a group.
Jax is uh... You've got a point there as much as I like Jax, but he gets free tank stats for just building damage items, all you need is some extra HP and maybe a QSS.
Lee Sin has burst early, but his damage drops like a rock later on if he builds tanky, he become more for the utility of his ult than actual damage.
Morde is weird, and I don't know him well enough after his rework to really have an opinion.
Renekton is the same as Lee, crazy early damage, and then drops of later on.
Volibear, now there's a fun one. Volibear actually doesn't do a ton of damage in bursts. Yes, his bite can do a lot of damage, his ult can let him soften up up to 4 targets at a time, and he can toss people around like they're paper bags full of balloons, but he has to be horrendously fed and ahead to 100-0 somebody on his own without building extra damage. The bite scales on HP, and executes, a lot of people say that it is too strong, but when you look at the rest of his kit he lacks so much damage to make the execute mean anything in a non trading situation. He needs somebody else dealing damage to make it scary, he can finish a job quickly, but only if someone is already low on HP. I don't know of a time where Voli can walk up, slap somebody who was full HP 3 times and bite them for a kill, it just doesn't happen, you have to do something wrong to get trashed by Volibear, if you're on 1/3 HP and he's slapping the minion wave, don't stand close enough for him to flash onto you and bite your head off, you're doing the one thing you should never do near Voli, which is be on low HP and within range. To sum things up I will quote Voli himself, "A fool and his head are easily parted". Just don't play into his hands, er, paws, and he's not a huge threat if you're on even ground.