I place Garrus and Tali on the top of my list, because they're like those two best friends who have been there with you through everything, have seen everything you have, known everything you've done, yet stand at your back and never question your motives. Plus, I love hearing them interact with each other when you take them on missions with you. As a male Shepard, I usually end up romancing Tali just because she's so amazing.
Next would have to be Mordin. I thoroughly enjoy his very A.D.D.-like personality where he's constantly just running through all of his thought processes out loud and whatnot. Unfortunately, my last time playing through ME2, I made a fatal error where Mordin ended up dying in the Suicide Mission. I forgot to send him back to the Normandy with the other surviving crew members and left him with the rearguard, taking Tali and Garrus with me to face down the human Reaper.
Next on my list would probably be Wrex. I love it in the Citadel DLC when the Shepard clone's mercs ambush you in the C-Sec shuttle, and he just drops out of nowhere and is like "Sup, Pyjacks??" and then just takes them all out. Plus, having him as a crew member in ME is pretty awesome. And then as he ascends to the leader of Clan Urdnot and eventually rules Tuchanka is pretty awesome. Shoutout to Urdnot Bakara as well, (Eve, for those who don't know her real name - spoilers) as she has such a wisdom and level head as you've never seen from a Krogan. Even Wrex. I will say it's disappointing that you never really get to bring him back aboard the Normandy as a permanent crew member again though after the first game, but I understand why.
Then there's Grunt. I feel like Grunt is kinda like Garrus in that he's that bro type who will stand at your back no matter what happens, and he's always there to pull you out of the fire when you need it. He packs quite a punch, seeing as he's Krogan. I love how excited both he and Wrex get when they see Shepard. "SHEPARD!!! -pushes everyone out of their way- Shepard!! -shakes hand happily-" Always warms my heart.
Liara is a great character, I feel. I love seeing her go from a "I'm not at all familiar with humans and have no idea how to conduct myself around them" type of awkward person to a very knowledgable and lovable character. I usually end up romancinc her a lot both as female and male Shepard. She just has that...gentleness about her and in the first Mass Effect, her awkwardness is kind of really attractive.
Say what you will about Zaeed, but he has his good points. He knows how to kick some serious ass when it counts, and his war stories deserve listening to. He can be a royal pain, especially in his loyalty mission. If your paragon or renegade scores aren't high enough, you can't save the refinery workers AND keep his loyalty. For who his character is though, I feel he's very well-adjusted and makes for a good addition to the Normandy crew.
Now I saw some of you place Jack even lower on this list, but I personally am a huge fan of Jack. Idk, I'm kinda drawn to her character because of where she's been and what she's seen. How she's perservered, and how she's ultimately better for it. She's not only an incredibly powerful biotic, but a capable warrior and especially fun in ME3. I also enjoy romancing her from time to time, because I just really love her personality.
Now, I hadn't planned to mention any of the more minor characters in this list (i.e. Chambers, Traynor, etc) but I feel like Kenneth and Gabriella from ME2 deserve a nod. Unfortunately I made another fatal error in which I waited too long to go to the Collector Base after they raided my ship, so Gabby died and the only one left was Kenneth. I do thoroughly enjoy them as characters and am always more than happy to pardon them for their involvement with Cerberus and welcome them back aboard the Normandy.
Miranda has slowly wormed her way into my heart over my many playthroughs of Mass Effect. She starts off as an ice queen, but her character develops over time into someone you can, at the end of the day, relate too and appreciate. She's not my favorite crew member, but I do highly value her as such.
Next would be Ashley. I almost never leave her to die on Virmire because I like her so much better than Kaidan. She's your typcal Alliance soldier through and through, but when you get to know her, she has a softer side that makes you want to be her friend. Her judgement can be a little off sometimes though, like getting mad at Shepard for joining the Illusive Man (or, rather, using him), and then trusting Udina in ME3 (which even with my very first playthrough of ME, I never trusted him). All-in-all, she's in my top 10.
Now for Thane. I thoroughly enjoy Thanes deep spirituality in combination with his deadly skill. And his wanting to make the universe a better place and atone for the things he's done in his life make him an honorable crew member. I don't end up taking him with me on missions as much, but he still makes for a good squad mate when you take him out on missions. And when he dies, it's pretty depressing.
Jacob. Jacob would be higher on this list if I felt he played a more pivotal role in the series. I like him in ME2 especially because of the fact that, until you get some of your old crew members back, he's the one guy within Cerberus who's not really there on the Illusive Man's agenda, but he's there to make a difference for himself and help the galaxy (and humanity) in his own way.
Legion. Legion is a great character because of the way his A.I. personality never really changes from that of an A.I. Unlike EDI, he never really becomes more human. He's relatable because of the way he processes some things similar to how a human mind would, but he's still an A.I. and doesn't try to be more human. Plus, I get a kick out of how he refers to Shepard as "Shepard-Commander".
Now to EDI. EDI is a very valuable asset to the team, but I feel like they made her too human-like for an A.I. It's kinda cool that she and Joker end up having a thing for each other, but it just makes me wonder how something like that would work. Idk. She is a great member of the crew, but I almost feel as if they should have just kept her as the Normandy's A.I. instead of putting her in a humanoid body.
- Side note, Glyph is pretty great sometimes. Especially in the Citadel DLC when fighting the other Shepards mercs, Glyph is constantly trying to talk the enemy down so they might live by saying things like "Why do you resist? Maybe we can talk about this" and nonsense like that. It just cracks me up every time
Now Kaidan. Kaidan has his good moments, but for the most part I find him just too stiff and too...prickly. Like..his personality makes him less approachable than some of the other team members of the Normandy and while his combat skills are admirable, I just don't really like him as a character. I feel bad about it every time, but I always leave him to die on Virmire and save Ashley.
I left the last two on my list as Samara and Kasumi. Kasumi, only because you get very little interaction with her one-on-one aside from her loyalty mission, which is always fun. I feel like they could have given her a more active role in the game, and it would have been great.
Samara is a very stereotypical matriarchal Asari. Wise in her old age, and formidably powerful, she makes a good crew member, but I also feel like there could have been more interaction with her aside from her loyalty mission an the occasional conversation or two with her.
I hope I didn't leave any of the major characters out, but if I did, I apologize for forgetting them.