TL;DR: Mana Drain & Esper Sentinel
Hey all! So, I was thinking, I'm warming up to the idea of the 'game changers' list, as I realized I have my own mental list of cards. Cards that as soon as I see the first or second one, I immediately re-evaluate the deck I'm playing against, as I realize it's a stronger deck.
But of course, my list doesn't align perfectly with what's on the published one. So I thought I'd push this out here today & see - hey what cards do you see that make you pause & go 'Hey, that's a strong deck'.
For me, the top of my list is probably any free counterspell (which they have), followed very closely by Mana Drain (which oddly, they don't). That's followed very closely by seeing any of the 'tax cards' (Smothering Tithe, Rhystic Study, Esper Sentinel, & Mystic Rhemoraz).
And I'll put a special mention out there for Staff of Domination. Which is covered by the 'no 2 card infinite combos' rule. Fine in isolation, but it's rarely in a deck without being exploitable. I just have a lot of friends who run it for that exact reason, so I see it a lot.
And of course, there's plenty of commanders that hit this as well. Atraxa, Ur-Dragon, & Edgar Markov come to mind. Fortunately they don't come up too often (for me), they seem to already have this unofficial label of being C-EDH only.