What ulti has the most impact in any given game regardless of team-comp (or title)?. Standalone ultimates. No combo with other champion ults or abilities (like Yasuo ulti). Not dependent on many "if-s" (Yi ulti is a great example "if he gets x kills it's the strongest" or "if he does not die" or "if enemy team has no CC"...).
Oh you wanted to do damage in burst meta? Have fun with cooldowns. Oh and... you're dead.
and the new 
ult arguably have the highest POTENTIAL impact if they hit 5, 
and 
get honorable mentions in this respect as well.
Hard to disagree w/ the guy who said that ult used intelligently is super impactful.
R will always be strong.
And A thought no one else had: 
Ult, because to some degree it warps the game and forces the enemy team into sweepers or other vision denial.
Nocturnes ult TFs ult
Malphite Amumu Taric Aphelios
Orianna ult. As soon as orianna is locked in, the team fight advantage is in ghat teams favor entirely because that ult is a game changer.
good question. i think anyone with aoe hard cc is pretty much tied.
I'd probably say gragas. Once he presses R all bets are off the table until its done and there's no telling how it's going to affect the following play until it lands.
A well time ult from one of these is GG
The most impactful ultimate...
I mean any champions with 5 free seconds of damage.
My eternal nemesis when on the enemy team, does not matter which role i am playing but when i am going for kills and´s its even remotely even she will ult, and it usually screws me over somehow.
Second option would be 
.
just gain so much presence through their ult.
Not only becuase of the semi global dash, but the global nearsight is insane and very underated
, the mix of damage and slipperiness makes her insane to deal with 1v1 , she has the potential to get a 1 button penta, the damage gets just so insane vs multiple targets
is a catch all case, it's strong to catch out targets, it can devastate teamfights, it can setup combos in a 1v1, it can be used to disengage, it's just very versatile.
With a good player
If they can hit it at the right time and place
If they get to exist to the middle of a teamfight
No one has mentioned Bard because he's unpopular partly because his ult is hard to perfect in gameplay. However, when you do his is easily the most impactful. You can disable towers, specific enemies, specific allies. Use it to freeze enemies for sales to position for a kill, pull half a team it of the fight etc. It's hands down Bard imo. Not even a contest.
Edit: my bad, one mentioned him.
global damage + shield + aoe + vision
.
And a cherry on top,
Wait, this looks like an actual teamcomp LOL
ofc. Control wards and Oracle might help a bit - but is someone play Teemo in minefield mood - it's close impossible to clear all of them. Cause of his ultimate - Teemo seems to have the highest champ for a comeback/win - while being a part of a weaker comp or a weaker team (late game scores like 14 - 38). For this reason - this is actually the most hated ultimate in League of Legends. Lost games that seem like free win (bot, mid and jungle - were on the loosing side) - and won games where half the team was spamming FF since min 15 (seemed like a sure loose at that point) - cause of a minefield Teemo. A low skill passive ultimate that can kill you - without even being focused (Teemo could farming at that time - in the other side of the map). Unrivaled since 2009...
In terms of late game teamfight. Urgot - removes the tank from the fight even at 25% hp.
EDIT: Wait, forgot, no one plays tanks anymore. :P
i'd have to go with 
a chance to undo what was done.
