D&D 5E (2014) Critical Strike

[Tapkill]

Is anything uncritable anymore?

 

[Prakriti]

No.

Adamantine armor grants immunity to critical hits, but no monsters have it unless the DM decides otherwise.

 

[the Jester]

Is anything uncritable anymore?

Not by the RAW, but there's no reason a custom monster here or there couldn't be if a DM wanted to do that. Personally, I love crits; I don't know that I'd use a simple "immune to crits" creature.