Although Mordin is probably my favorite character of the entire series, Garrus and Tali were my original favorite squad mates way back during my first ME playthrough. I guess it's because although I liked the others, these two gave me more insight into the Mass Effect galaxy than the others during missions. In the first game your companions made a little "hmm" noise if they had something of interest to say at certain points, such as around the Citadel. It was these moments I came to love but Ashley and Kaidan were quite human-centric and justifiably saw things from that perspective - asking rhetorical questions rather than answering. Wrex was always funny but had a habit of dismissing things of interest "who cares, let's kill stuff." Liara, tbh, always annoyed me although she has grown on me since.
Garrus' development is quite interesting. We know when we first meet him that he hates injustice but it isn't until we talk to him about Dr Saleon that we realize just how far he'll go to catch the bad guy. He readily admits to police brutality during interrogations and is ok risking civilian lives in order to see justice done. This single mindedness is something that raises it's head again during his time as Archangel, as well as his quest to find Sidonis. This dark side to a character that is otherwise quite calm and wise is what I find appealing about him - he goes from passionate but somewhat naïve detective to Reaper Expert of the Turian Hierarchy without losing that flaw.