Worst?
1. Kai Leng. No I do not care how he was presented outside the game, in supplemental material. Like Plinkett's review of the Star Wars prequels, it should be based on his presence in the game. As Plinkett also said, "Describe the following Star Wars (Mass Effect in this case) character WITHOUT saying what they look like, what kind of costume they wore, or what their profession or role in the movie was. Describe this character to your friends like they ain't never seen Star Wars (Mass Effect)." Good luck doing that with Kai Leng.
People like Kai Leng do not exist. Some people want to watch the world burn, buy they are loyal to no one and he was loyal to TIM/Cerberus, and those people may have a messed up past, childhood or something that f'd them up. There are many things that could have been done with the character to make him interesting. They could have made him complex. Example. Past relationships or story arcs with Miranda or Jacob. Him losing a loved one, or a child and TIM using the Lazarus project as enticement for services. If not complex? Consistent. If you want to just make him a "watch the world burn" guy? Have him attack TIM or have no loyalty towards him or go rogue.
Kai Leng seems like he was added to the story to appeal to anime fanboys and girls (not that there is anything wrong with anime), sword and all. It seemed like a box on a list of "appeal to as many people as possible" (same reason Star Wars prequels failed) to check off and nothing more.
2. Diana Allers. Should have been Emily Wong. It is almost like the initial script/draft called for Emily Wong and she was put into the game to pander to some game journalist outlet or something... What makes it worse is that we got a message from Emily in ME 2, and I really wanted to talk with her, but that didn't happen. Emily and Shep had a backstory, and it could have been a legitimate romance arc. It could have been REALLY good.
3 and nowhere near as bad as 1 and 2 on the list. Liara in ME 1. I hit the spacebar. The fainting is just painful to watch. Liara in ME 2. No complaints and much better done. ME 3? Back to annoying me, with too many scenes, but at least not as bad as ME 1. If I hadn't disliked her so much in the first game, I might have liked the character a lot more in 3.
4. Cortez. Hits on me at a bar when even the slightest bit of asking around, would tell you I am in a relationship already. So apparently he is either really stupid AND takes sex casually, AND has zero respect for my LI, so he is a bad person, yet Bioware wants to paint him as the opposite of unselfish for the rest of the game. Cries to me, when Vega was his best friend and that conversation would have happened while drunk at a bar (we saw they both loved sports and drinking in the Citadel DLC) with his best friend (that is what guys do). Also his best friend was sitting over there doing pullups, ignoring him, listening to the message from his boyfriend (probably not the first time), just so Bioware could force me to have this awkward conversation with someone I barely know, when this guy should have went to see a grief counselor, if he could not even tell his best friend. Completely unbelievable as a character and a person. Another check on a box for possible buyers.
5. Ashley ME3. Defeatist, when she had no reason to be defeatist. If anything she should have been stronger for "walking through the fire". Every time I entered that room, I was like oh great,"Debbie Downer" time. They kind of fixed her with Citadel DLC where she punches the guy with Shep. Problem? That was DLC.
Runner up. Edi in ME3. Seemed like a ridiculous plot device to make us choose Synthesis, with forced paragon acceptance of absurdity. The romance was ridiculous, and Javik was right. Joker needed his head examined. Why it is a runner up? That voice actress has got the greatest voice I have ever heard. I would listen to her name the ingredients of bubblegum on repeat.
Vega did not bother me. Why? Cus people like Vega exist. Just because people would not choose him as their best friend did not make him a bad character to me. He was believable. Nope, I never really took him on missions, but I did not hate the character.
Starkid? Does not exist. MEHEM mod removes that abomination. That was all a bad dream.