Not my headcanon choices, just the game choices? Okay
Backstory
Male Shepard
Vanguard
Colonist
Sole Survivor
ME1
Did all the random sidequests, retrieved the matriarch writings, salarian medals, and whatnot
Spared all of the colonists on Feros, as well as Shiala (Honestly, wasn't their fault)
Spared Ethan Jeong (Last thing the colonists needed were crazy suits making life hell)
Scanned the Keepers (too creepy not to learn about)
Worked with Gianna Parasini (seemed to be the best choice)
Made the serum to save the Noveria folks (in case they were wrong and the bioweapon was contagious)
Saved the Sirta Foundation folks with no deaths (One of Shep's greatest challenges)
Saved Major Kyle (He needed to be executed, but the biotics could be disbanded peacefully)
Saved Toombs (Shepard felt pity for the guy)
Saved Talitha (by convincing her to take the pills herself)
Stopped the VI on the moon
Killed Wrex on Virmire myself
Sent Kaidan with Kirrahe
Left Ashley on Virmire
Rescued Liara on the last mission
No Romance (Liara was the only choice, and she was creepy)
ME2
Did all of the random sidequests (Cerberus paid, and Shep needed all the resources he could)
Recruited all 12 squadmates (needed the best of the best)
Did not let Miranda shoot Niket (She would have been distracted by it)
Let the feral colonists kill Jacob's father (What he did to them all, the men and the women, was so sick that he deserved it)
Stopped Garrus from killing Sidonis (The way turian society works, it was the better choice)
Let Mordin shoot Maelon (Guy was completely deranged, and might try something crazier)
Destroyed Maelon's data (Resurgent krogan was the last thing the galaxy needed)
Made Grunt part of Clan Urdnot (Focus and control is necessary on a high stakes mission)
Spared Aresh (Letting Jack shoot anyone she pleased, coupled with her temper, would cause problems)
Cleared Tali by shouting at the Admirals (Because the whole thing was total bull)
Killed Joram Talid (Racists like him prove Ashley right, and there'd be no changing him)
Killed Morinth (There was no way she could be trusted)
Spared the oil refinery workers (That's what we were there to do, Zaeed doesn't get to change mission parameters. I forced him to accept my terms)
Shot the pot in Hock's vault (because the damn thing was ugly)
Destroyed the Collector Base (TIM would never have shared the data)
Went through Lair of the Shadow Broker, not exchanging a word with Liara at all
Sent David Archer to Grissom (even if Gavin was good to him, TIM would never be)
Did not warn the batarians (Would have been pointless to try)
No Romance (Miranda wasn't his type, It would have been too dangerous to try for Tali, and Jack was too unstable)
No deaths on suicide mission (Because any deaths would have been due to poor decision making)
ME3
Brought Allers on this ship (Just ignored her the whole time)
Sabotaged the genophage, saved Mordin (Told him Wreav was worse than the Reapers)
Kaidan sided with me without question (Because capable leaders handle any problem in their offtime)
Kaidan shot Udina (He's gotta make big-boy decisions if he wants to be a Spectre)
Saved Samara and Falere
Sided with Xen and Garrell on arguments, but punched that bastard for shooting me down
Made Peace on Rannoch (That's the right thing to do)
Didn't pout on Thessia (Just went to the next mission)
Let Miranda defenestrate her dad
Headcanoned an entirely different ending
No romance (Shep likes the ladies, Liara is still too creepy, and he'd forgotten Allers existed).