I only joined the community at the end of 2014 so I didn't see most of them happening, but here are the ones that impress me the most:
Jacob's 1'58"647 MC: An amazing improvement, Jacob pulled 2 amazing laps off. Maybe it would still be lasting if Sun had not discovered a new glider strat.
そうめん's 1'34"811 rMG: An obvious choice. そうめん literally destroyed Adro, by cutting over 600 thousandths in only 4 days... It's still the no big jump BKT if I'm not mistaken, which is amazing considering そうめん only did 2 bridge cuts and set this time in early 2013.
Aiguille's 1'53"965 WS: I don't think Aiguille himself was expecting to directly improve his 1'54"189 to a 1'53 time. He managed to align 2 BKSes in the same run (although the 10-0-0 coinstrat allowed the lap 2 one to be replaced with the lap 3 one), some of them are still standing. Over 1000 days after, the WR remains unbeaten and is still arguably the best current WR. Simply outstanding.
AdroMaster's 1'54"358 rAF: A great improvement of what was already one of the best WRs thanks to the tough competition between Adro and Branlo. It had some flaws, but the rest of the run largely compensates them. It stood 243 days, which is definitely mind-blowing for a 2012 WR. And it's even more impressive considering that the next WR, a .047 improvement, also stood 294 days.
そうめん's 1'11"874 rMC2: Maybe this isn't very impressive anymore, even as a non-pasta time, but this time was set in early 2012, only one month and a half after the game's release. On one of the shortest and the most precise course, そうめん has been able to beat Mick's previous WR by over .100 despite missing an SMT. Even to this day, his time would rank #69 on the PP despite the discovery of an OP strat aswell as the fact that 5 years have passed since.
mume's 1'47"947 PPS: Although some people might disagree, this WR is still one of the most impressive to me. After stealing the WR from るえもん in only a few days, mume came back later to reach a sub that even top players could only be dreaming of. And he did this directly from his previous 1'48"201 WR. I know he only has been able to reach it thanks to strats he implemented, but the fact that he has been the only one to notice that for once, Sun was right about using Metal Mario on a track (lol) is another proof of his talent. Moreover, even if this is the first WR implementing this strat, only one player has been able to improve it since and only by 6 thousandths.
Michael F's 1'43"537 rLR: Another early WR, yet another awesome one. Set on January 7, this WR lasted 72 days and allowed Fried to remain ahead of anyone else by a fair margin. It still ranks at #59 on the PP (Fried apparently improved it to 1'43"535 in April, but it doesn't change anything) despite the fact it was using the old 8-2-0 coinstrat.
こ~ちぇ~'s 1'47"646 MP: This one is a bit special. It's not especially strong in itself, but it's the first I ever watched. I had 1'58 at that time, thanks to a 1'49 ghost from Lee that I could barely follow. However, racing a 1'49 ghost has nothing to do with watching a 1'47 run in first person view. MP being a track with a lot of action, I can't find a more appropriate word than "mind-blowing" to express what I thought about this run. Bouncing off the back of the notes, going down to take coins, releasing drifts really fast, making very tight turns... Seeing this is truly what made me want to play this game competitively.
Guess I'm gonna stop here cause I'm tired of typing

However, I feel like some WRs are only impressive because of the WR streaks they represent, and those streaks definitely deserve to be mentionned too:
- Aiguille on WS
- Jacob on BC
- MVT on RRM
- おまえモナー on RR
- Coco on rKC
- Mr.L on rRR
- mume on rWP
- Liar on MP
- ZCrone on CCL
- Ray on NBC G