Swissdictator:
At 'ard boyz today, against Empire my mostly Nurgle Daemons devestated his army by the end of turn 1… and Epidemus was in full effect (all upgrades) already…
Sanguinarian:
As a general rule I think turn 1 “wins” are usually closer to simply crippling the enemy army in turn 1 and thereby giving you a significant advantage over the course of the game. It’s still technically possible to pull back a game after having your army badly weakened or your undead general slain in turn 1 it’s just very very unlikely 
As far as I’m aware there are a few methods of getting a “true” turn 1 victory, but they’re pretty unlikely. The scenario where you begin 12" away is one possibility. Also Orcs and Skaven have spells that can potentially hit every unit in the enemy army. Absolutely insane dice rolling would be required of course and it’s less achievable for the skaven (as each unit can only be hit once). Another method is a highly manoeuvrable death wizard versus nicely lined up low I troops.
There was one army back in the Storm of Chaos expansion that could potentially win the game before it started! The High elf list was able to shoot with its archers and bolt throwers before the game started (without line of sight, range or modifiers being taken into account IIRC). If this was used against something like a wood elf army with lots of low T unarmoured troops, it wasn’t infeasible to kill off all or most of the army before turn 1.