Have seen quite a few clichés over the years, but adding to what's been said so far:
JFK Lives - Seriously, this one's been done to death over and over again. Granted, I can somewhat empathize with the "martyrdom" aspect that comes with a handsome, vivacious fella like Kennedy being taken out so unexpectedly, though digging into his various health issues and escapades over the years will tell you that's more public image than it was reality. That's something people taken in by the idea of a two-term Kennedy presidency forget, I imagine — especially since future battles over civil rights and the brewing counterculture of the Sixties would've tested his metal.
Nazi Super Science - Basically, the idea that Nazi Germany was this cutting-edge, technologically supreme force of destruction that could've easily won on the back of its technological superiority. Really, it always amuses me how even non-Nazi apologist Wehraboos active in the AH community hype up the state of World War II Germany's science and military technology — despite how misleading and cherrypicked their thesis is. For one, the Wehrmacht relied on horses and captured vehicles to lug troops and supplies around, not to mention French tanks actually being
superior to German ones pound for pound — the French commanders’ incompetence in putting their superior armor to good use notwithstanding. Never mind some of the really dumb and wasteful “Wunderwaffe” projects Hitler blew the Reich’s money and best minds on, like the
Landkreuzer and other ill-conceived mechanical monstrosities of the sort. So, hyper-futuristic victorious Third Reich? Yeah, how ’bout no!
Reverse Cold War - A wholesale "swap" of the Western and Eastern blocs' economic ideologies and systems of government over the 20th century. Usually, it's the US undergoing a communist revolution of some sort, while Russia liberalizes and becomes the world's foremost bulwark of "God and tradition!" in facing off against the Reddened West. Most well-known example of this scenario I can think of is
Back in the USSA, though there are certainly online TLs — such as AH.com's
Hammer's, Sickle's, and Mushroom Clouds — that are arguably even sillier, especially since they go full-tilt on direct analogies and ATL doppelgängers of OTL events, factions, and key figures.
Missed a good number, no doubt, but will give others a chance to speak and leave it at that for now.