I'm going to be the boring one and say Kaidan, because he's a beast on the battlefield and when I have him around, I wouldn't really need any other squaddie as he can do completely everything I might wish for - be it biotics, tech, crowd control, disabling charging krogan, healing, hacking, crate opening, etc. I also enjoy his personality because he's a caring, empathetic and smart character that can easily serve as my Shepard's moral compass, but can also be pretty funny and sassy at times and I think that what makes it even more funny is that it sometimes comes completely out of nowhere and in total deadpan. He tends to give advice and insight into the universe, various situations and dialogues. He tells you when you are out of the line or when you do something stupidly naive, but he also supports you in various conversations. He also has an interesting backstory and you can easily tell how it influenced who he is.
I agree about Tali. I think it's perhaps easy to overlook how badass Tali actually is because she doesn't really flaunt it about and, when on the Normandy, is largely a walking codex about quarians and geth. She is also really sweet, has some genuinely interesting things to say, and I find her kind of easy to relate to in some things. Another thing is that Tali is very young. One of the youngest people in the whole crew. She is also a civilian. Yet when you meet her for the first time, she handles herself like a boss. In that scene, Tali proves she is a survivor. She reacts in the blink of an eye and knows exactly what to do when faced with an overwhelming force. If that's not cool, then I don't know what is. I also think she has more layers that aren't immediately visible, because if you drag her through the missions, you get some interesting insight into her character. On Noveria, you can see she may not be such an innocent cupcake she might seem to be at first. I find that great. Tali definitely belongs to my favourites.
I also often take Liara with me on missions. It feels really weird to have an archeologist who had probably never held a gun before along on military missions, though. In many ways, she still feels like a kid. Much more so than Tali, in my opinion, despite being much older than her. It kind of feels like giving a pistol to a kitten. However, I love her skillset. She's incredibly useful on the battlefield and at crowd control. I also must say she's pretty cute, personality-wise, and I really like her voice and the fact she plays an important role in the game's story.
And Wrex. Wrex is definitely awesome. The krogan are probably my favourite ME race and Wrex is a lot of fun to have along. He can be quite aggressive and ruthless, but also surprisingly wise... and all that sass. Sometimes his comments are hilarious simply because he's so brutally honest. I greatly enjoy his attitude, his stories, and his Virmire dilemma makes him quite a deep character. He's the team's juggernaut that can do some serious shooting and that can do a little bit of biotics, too, so he's definitely useful to have.
To be honest, I like all of the companions. But Kaidan is pretty much permanently in my team with my favourite Shep and I mostly tend to combine him with Tali, Liara, or Wrex. I take Ashley and Garrus, as well, sometimes and I like them, too, but I probably use them least.