First of all, Events aren't cards, they're "card-shaped-objects" in semi-official DX canon.
But more importantly:
define it how you'd like
makes this entire discussion virtually meaningless. At the very least, it requires a re-framing.
If you are allowing people to use their own criteria, then you are not looking for the "Greatest" card, you are looking for the community's "Favorite" card, a purely subjective measurement.
In which case, Roadrunner is entirely reasonable in nominating Scout because there is no objective criterion by which it can be dismissed. It is his favorite card.
Any discussion where that makes sense definitely needs some tighter ground rules.
Also, I would submit that the Ace of Spades is the greatest card in the majority of traditional card games. The Queen of Spades is only the highest ranking card in a few systems, such as the point values in Hearts, and has special meaning in Pinochle. Most other games put Ace as the highest rank, and most games (such as Bridge) rank Spades the highest suit. (Shout out to Duplicate Bridge players. And no I'm not talking about gaining a second copy of a cost-reducing action, or playing two cost-reducers in order to gain a second copy of Province.)