Eh, I think Zinogre, while still not too challenging, is actually still one of the harder monsters in the game if you do him solo. I feel like if I'm to solo monsters in the highest rank, the only ones that put up a challenge are Diablos, Nargacuga, Goss, the final boss (because I've fought it very little), and to an extent Zinogre given how it can punish the player if they aren't careful (which is how I play most of the time lol). However if you're doing multiple players at once, then the only one that is really hard is Nargacuga, and maybe the final boss (Diablos might actually count here too).
For me, by far the hardest is Nargacuga though. Which is a little weird given how I always heard he was one of the lower tier flagships. He feels like the only one who can actually kind of match a 4 man team, given his quick swipes and constant moving around, as well as his compact-but-still-large size, means he's always bound to hit someone usually.
In general, I think the biggest problem this game has, and something they'll probably address in the expansion, is that while monsters certainly do a lot more damage than they did in base World (at least from what I remember, base World was very unpunishing, and this is a welcome change), monsters in multiplayer don't seem to account very well for how much stun multiple players might inflict at any given moment. About half the match in any multiplayer game becomes ping-ponging the monster from one stun animation to the next. Hopefully the elders in the update don't suffer from this.