Is ultimate tyranno forced to attack all opponents monsters?

No, Ultimate Tyranno does not have to attack all opponent's monsters.

Here's why:

* Ultimate Tyranno's effect is a "Must" effect: It forces you to attack with Ultimate Tyranno during your Main Phase 2.

* The effect doesn't specify attacking all monsters: It only says "attack a monster" - meaning you can choose one monster to attack.

Therefore, you can choose to attack just one of your opponent's monsters, even if they control multiple.