I love Rei, Sailor Mars. When I was like 12, I didn't even really like her, my favorites were Usagi and Makoto. But now she's definetely my favorite.
But there is something about all of them that I like very much. ♥
As of this point in time, I've only seen the first season, so I may be a little lacking in proper knowledge, but my favourite Sailor Soldier's got to be Mercury/Ami.
She's a lot like me, personality-wise, so much so that, at a Japanese culture convention held in my hometown last year, my first ever cosplay was as her! (you can see how I looked in my profile picture)