I also vote for beholder.
Tempted to say illithid (mind flayer), but as other have said, it is a specific feel and cthulhu vibes that doesn't make for the best branding to be the mascot for the game.
A red dragon is a good choice, even if not unique to D&D, "dragon" is in the name, the logo is a dragon, and off all the dragons, it never feels out of place and instantly just works as a mascot for D&D.
The displacer beast and mimic are great choices that feel unique to D&D but I don't think they have much recognition outside of those who are already fans. Now, I get that the beholder may not be any more recognizable to non-fans, but it just sticks out as something different. I thought it worked very well on the cover of the 2014 Monster Manual.
I know that the OP isn't asking this in terms of what should be on the 2024/5 monster manual, but if I had to guess, I'm almost certain that a dragon will be on the cover of the new MM. If there are multiple monster, and I assume that there will be, it will be the biggest bad in the background. The covers of the PHB and DMG both show creatures at multiple tiers, and I could see a similar cover for the MM where you have some lower level monsters in the foreground, mid-tier monsters in the middle and an ancient red dragon in the background over seeing all of them. The PHB and the DMG both have a dragon that figures as the most powerful creature amoung the characters depicted. A gold dragon on the PHB, a dracolich on the DMG. So I am guessing an ancient red on the MM.