Which of these iconic Dnd Villians has the worst stat block?

One thing I've always found weird is that iconic dnd villians sometimes have lackluster or badly designed stats. Vote and tell me your reasoning in the comments.

5e in general has bad boss designs, but Strahd and Tiamat are at least functional, Vecna really isn't.

Vecna is both my least favorite of these as a boss monster and my favorite of these stat blocks overall.  As a boss monster, it suffers from passivity - the lack of legendary actions and making its off-turn abilities simply reactions to player actions makes for a full fight where the boss hardly feels impressive and in control.

But as a regular monster stat block?  Something I’d throw at a party for a random dungeon room or as a secondary monster to the real boss during their 3 or 4 play?  I actually really like this design. In those situations, this design feels powerful and helps regain parity against players’ action economy, without feeling “legendary” in a way that might make the random side creature feel more important than it is. 

Quote from Caerwyn_Glyndwr >>

But as a regular monster stat block?  Something I’d throw at a party for a random dungeon room or as a secondary monster to the real boss during their 3 or 4 play?

"Makes a good minion for a more relevant villain" is a sad thing to say about a nominal CR 26 (I say nominal because Vecna doesn't really perform at CR 26), but I wouldn't even call him a good secondary monster stat block, because of his vile teleport mechanic. Monsters with high levels of (possibly preventable) healing and low base health generally underperform, but will sometimes wildly overperform. If I were using him, I'd delete the healing from vile teleport and give him a different durability mechanic (could just be more hit points, but I'd be inclined towards something like an extra life -- the first time he dies, he reforms).