I think the reason why everyone is giving similar answers and why possibly you can’t think of one yourself is that RDR2 is designed as a cinematic, narrative experience where no single mission is designed to be much harder than the other (except for the collectible based stuff naturally). They’re all kinda tuned to be “middling” difficulty so that the player doesn’t get bored but also isn’t seriously impeded from continuing the main story.

It’s also a mega long game that the majority of us haven’t likely touched in great depth since it came out 4-5 years ago. The dedicated subreddit might know more as those folks will be more invested into the game.

If you still want an answer, the attack on the O’Driscolls (I think that’s what they’re called) while you’re still in the snowy area at the start. Unfortunately for your purposes it’s for no other reason than being a mission with a lot of enemies while you’re still getting used to this amazing game’s, awful controls .