All right, here's my rankings, just for ME2 and ME3, ME1 just needed someone with enough skill to open the damn boxes (although I liked Ash and Kaidan's Citadel sidequest banter)
ME2
-High Tier-
1.) Mordin - Absolutely hilarious, fun to listen to, fun to talk to, utter fantastic story
2.) Legion - Compelling character, really made me think
3.) Zaeed - He's just fun!
4.) Samara - Of all the characters, she's the most alien. I like that in sci-fi games
5.) Thane - He was cool, and his philosophy, although not as alien as Samara's was very different
6.) Kasumi - Very cute, bright, and bubbly. Everyone else was doom and gloom, and the contrast was helpful
-Mid Tier-
7.) Garrus - I liked him, but there wasn't enough of him. His story was interesting, though
8.) Miranda - She started out strong, but I wasn't as interested in her loyalty mission, and she kinda just fell into the background.
9.) Tali - The supporting cast in her story (the idiot Prazza, Veetor, and Kal'Reegar) carried it more than she did. But her romance dialogue did give her some depth, and her context in her loyalty mission was nice.
10.) Grunt - He was kinda fun to hear his childlike battle quotes, but I didn't care so much for the constant talk of krogan
11.) Wilson - Massively underutilized. I did want to know more about him.
12.) Jack - Violent thug with no real backstory outside of romance. The paragon interrupt to stop her from killing Aresh was the only real interesting thing.
13.) Kenson - Combined all the exposition fairy parts of Jacob in something I had to pay extra for. Fun to kill, though.
-Low Tier-
14.) Morinth - Sorry, but Samara made too much sense when she pointed out that Ardat-Yakshi can't be the destiny of the asari because they are sterile. Other than that, just blue vampire space babe
15.) Jacob - The interesting parts of his loyalty mission had nothing to do with him (Ronald Taylor was just someone you could love to hate and leaving him to the feral males was cathartic). Other than that, he was Navi without the blue wings.
16.) Liara - Some foisted blue (expletive redacted) I had to pretend to care about and be nice too. Why couldn't I kill her?
ME3 (not including Arena characters)
-High Tier-
1.) Garrus - Finally got the buddy aspect down
2.) Kaidan - Moved on past Horizon, had some utterly fantastic and sometimes heartbreaking(or warming) dialogue, but he dropped off after the second half of the game.
3.) Tali - I felt for her more, putting her on Rannoch gave her some much needed context
4.) James - He didn't do anything to aggravate me in the vanilla game (except for a little too much krogan worship), and ultimately, I thought he was a fun guy. Freddie Prinze did a good job.
5.) Anderson - I'm a self-confessed Keith David fanboy, but Anderson's character was very, very awesome.
-Mid Tier-
6.) EDI - Absolutely, positively loved her character development and interactions. Absolutely positively hated the sex bot aspect. It cancels, for the most part. If EDI needed a body, it should have been more like her personality: A rabbit that specialized in trollface
7.) Ashley - She had "some" interaction, that was good until it got ruined.
8.) Javik - Yelling and criticizing Liara is aces in my book, but the dialogue with him was too black and white. You either supported his "millions die" philosophy, or you were too ideal. Some middle ground would have been nice. Still, agreed with the "weak links hold us back" one
9.) Nyreen - Massively underutilized and her story read like a 12-year-old's xeno/FemSlash fanfic. But I liked the aspect of her being a soldier and discriminated because of her biotics. Now if only Garrus/Kaidan/Ashley got to talk to her about it
-Low Tier-
10.) Wrex - Character shilled, massively annoying, fake best friend who defies physics for the sake of creating someone unbelievable. If my party is going to rib on each other, don't stop to give Wrex tons of praise. Forced Big Good in a way that defies all culture reasoning. Fortunately, easy to kill off. Even better, the story is richer without him.
11.) Aria - Before I got to the base, I was screaming at my TV: "Shut the (expletive redacted) up, Aria!" And she didn't, throughout the whole thing. The first paid DLC I ever deleted, and she was the reason why
12.) Liara - Forced best friend, constantly coming up to my cabin, not actually my friend, never get a chance to kill her without dooming the galaxy. Don't even get the option to call her on the deranged stalker she is.
Honorable mention - Miranda is High tier in ME3.