I am something of what you guys probably will refer to as a noob, though I have some experience in +50-70 minutes gameplay.
Just as of today I was playing Tristana (yes, it's my favorite hero). Can't really recall what characters my teammates had except Mordekaiser (tankbuild), and "The Barbarian King" who didn't do much else than feed Master Yi and Blitzcrank from start to end, and then lots of melee DPS.
Enemy team: Mastey Yi (Very fed), Blitz (Very fed, had around 4k hp and alot of armor), another tank that I can not remember the name of (blue color and a big shield, healing spell aswell, had a really heavy armor/magic resistance build) and then there was Twisted Fate (completely rubbish) and Fiddle (average) if I remember correctly.
I go for the Malady - Berserker greaves - Black Cleaver - first stage of phantom dance - the +80 dmg, +20% crit and 250% crit dmg-thingy - phantom dancer - Bloodthirster - sell boots and get the 40% attack speed and 40% armor penetration. After that it really was cakewalk. I had 8-6-13 or something at this time, but oh it would change!
They got all our inhibitors (purple as if it makes any difference...) And I stood back and defended while they pushed, sorta stupid, yes I know, since I threw away +800 crits with 2,5 attack speed (Rapid fire popped), 1.8**-something without rapid fire.
Anyways as you might have guessed Master Yi and Blitz slaughtered my teammates and it was me versus the entire team (all level 18 with all end-game items they needed, not saying it was a correct build).
They destroyed top inhibitor again and came at me while I was camping in-between the last two towers, both with almost full hp.
And here comes the part where you have to put the laughter in slaughter, I have no idea what happened: I just popped Rapid Fire and saw Fiddle going down withing three seconds and just rocket jumping into the middle of them, still with rapid fire and started picking them off one by one. They couldn't get me down since of 63% crit and getting back around 350hp everytime I critted. This totally turned the game around and I alone pushed them (creepskipping, yes it was a bold and ugly move, but hey! In love and war, everything is allowed!)
Managed to get down one and a half of their inhibitors and one tower. After that it was just for my teammates to march on in (I got killed once the other team respawned) but we had so much creeps in their base they couldn't do squat.
So I would nominate Tristana to be one of the best end-game characters.
Only other characters I have seen turn a game around like this are Ezrael, Twitch and Kennen, though I have never seen a 1v5 fight where the one win.
My five cents.
Also inb4: TL;DR
Hope you have fun while playing The Game and good luck.
Ps. My summoner was level 11 when this occured and had no runes or other of that fancy stuff. Ds.