Bioware has long stated that there is no canon in Mass Effect - this was mentioned in many interviews after the original game was released. One such quote by Casey Hudson says:
"We have a rule in our franchise that there is no canon."
However, based on the information available in the games through default choices made if you start a new character in Mass Effect 2 or 3, we can assume the following as the default story:
Mass Effect 1
Ashley or Kaidan isn't included in this list, see Mass Effect 2 for why.
- You recruit Garrus and Wrex
- Wrex is killed on Virmire
- The Asari Shiala is killed by the Thorian on Feros
- The colony on Feros is not saved
- The Rachni Queen is killed
- The Council is killed, with the ship the Destiny Ascension, during the attack on the Citadel
- Udina is appointed Counsellor
Mass Effect 2
Interestingly, during character creation in Mass Effect 2 you get the choice as to whether Ashley died or Kaidan, all in all this event is not that important to the story as it is more about squad build up.
- Tali is exiled from the Migrant Fleet
- Grunt, Legion, and Samara are never recruited
- Samara is killed by Morinth trying to escape from Ilium
- Suicide Mission Survivors: Mordin, Jacob, Garrus, Tali, Miranda, Joker and Dr Chakwas
- Suicide Mission Deaths: Jack, Thane, and the rest of the SR-1 crew
- The Collector Base is destroyed
The DLC is not considered completed by Shepard, but the events still occurred. This means that the Batarian System is destroyed, and Liara is the Shadow Broker.
Mass Effect 3
This section is largely open to debate and subjective opinion, however since there are a lower amount of choices to make in the third game, and many of the key choices leading up to the final choices in this game have already been made (ie: Wrex being dead already, Jack being dead already, etc) you will not be encountering these characters in the final game.
- Shepard runs around the galaxy for a bit
- Quarians survive
- Geth don't (Legion is already dead remember)
- Genophage is not cured and thus Mordin survives
- The RED ending, with Shepard surviving (for future Mass Effect releases, obviously)