It's tricky to pin down one single "most common" breakfast dish in Bhutan, as breakfast habits can vary regionally and between households. However, here's a breakdown of common breakfast elements and dishes you'll often encounter:
Red Rice Porridge:

This is a staple in Bhutanese cuisine.1 Red rice, a local variety, is often cooked into a porridge, sometimes with added butter and cheese. This hearty dish is particularly popular in the colder Himalayan climate.

Dairy Products:

Dairy is a significant part of the Bhutanese diet. Salted butter tea (Suja) is a very common drink.2
Cheese is also heavily used in Bhutanese cooking, so it is not uncommon to have cheese incorporated into morning meals.

Other Common Elements:

Simple breakfasts might also include wheat cereals, fruit juices, and sometimes potato wedges or Bhutanese pancakes.3
Aloo Paratha, and egg dishes are also eaten.

Ema Datshi:

While Ema Datshi is considered the national dish and is eaten throughout the day, it is not uncommon to have it during any meal, including breakfast.

Essentially, Bhutanese breakfasts tend to be simple and nourishing, often featuring rice, dairy, and potatoes.