I think that, if Hitler was killed, the new Fuhrer would've been, prior to 1941, Hermann Goring and, after 1941, Reinhard Heydrich. Goring was a fat drug addict and there would've been civil war. Heydrich would've been MUCH more dangerous for the Allies.
What I would've liked was if someone had killed Stalin in the 1910's or 1920's, before he began his reign of terror. If Germany tried to invade the USSR, there would've been much more competent Soviet Generals to oppose them and, quite possibly, they'd have been more successful in defeating the Nazis (or the Imperialist Germans who might've succeeded Hitler) sooner.
Yet ANOTHER scenario would be, if Hitler was assassinated after the fall of France, but before the Soviet Invasion of 1941, Germany would've gotten a competent leader who invaded the USSR in March/April rather than June 22. Not only that, but if they'd had treated Soviet soldiers and civilians humanely (the Wehrmacht was originally considered Liberators), then Stalin would've probably been taken out back, shot, then thrown into a ditch by August.
What I would’ve liked was if someone had killed Stalin in the 1910’s or 1920’s, before he began his reign of terror.
How about ganking young Karl Marx?
Best case scenario is, what if Hitler takes Moscow, and as a result, Stalin is overthrown (my guess is that Khrushchev very well may have been the one to engineer such as coup), and Russia still defeats the Nazis.