Hmmm, well I rotated sometimes just to get a kick out of the characters, especially on my second playthrough. On my first time through the game though, I mostly stuck with Samara and Thane - I found the (Mass) Throw ability to just be pure awesome, fun and most importantly, useful/powerful, especially when used with (Mass) Pull. Those two were also loads of fun and I really liked them as characters.
I liked Legion a lot and due to him being a sniper, took him a few times on the second playthough, but I have to say, I find Techies just useless. I was an Infiltrator so I picked up the first level of robot-mind-control thing myself and that was all I needed, cause I rarely ran into them and even when I did, I'd rather control one who'll distract and shoot at the others than take control of a whole group who will then (if there's no one else) just stand around picking their noses.
I found Miranda and Jacob far too boring/simple (why take boring "standard" humans over cool aliens when playing a space game) and Jack was a bit too mental for me and not too interesting. Grunt was cool but I wasn't into fighters and, like Legion, I found Mordin/Tali mostly useless as techies. Garus was cool though, I've had him instead of Samara/Thane (rotated) on my second playthrough and was pretty useful with the explosion thing when there were synthetics around.
Overall though, Samara+Thane is my favourite choice for both interaction (I love how "elegant" they both are as fighters and characters) and abilities.