Sailor Mercury is one of my favorites, so this is hard to chose. I love Ami in her civilian form. I'm always partial to the smart ones, and I especially adore her PGSM incarnation, where instead of being just the Smart Girl and the early moral center of the group (which are awesome in of themselves), she is also the dutiful daughter who is chasing her mother's dream and is also plagued by insecurities in wanting to have friends...and initially resisting her destiny to be a Sailor Guardian at first. I also love how she has blue hair, because unlike the rest of the five-girl-team her hair is a non-natural color and a visual reminder of her anime origins.
But, I absolutely love Sailor Mercury. I love that she isn't the strongest but that she plays the support role. Not everyone has to save the day, and I like that she helps out and uses her mind to figure out a way to get out of jams instead of just blasting through. However, her civilian insecurities also eat at her at times. I also love how in the only episode where she was a full-fledged teammate (not her debut episode) that she was a voice of reason without being a jerk like some other allies. I also love the blue on blue, as it is a very unique color pattern of complimentary colors instead of contrasting colors. Only Chibi-Moon's pink on pink comes close.
If I love Sailor Mercury, I love, love, LOVE Dark Mercury. One of my first fanfic ideas was to have an evil rival to Mercury since I couldn't canonically turn Mercury evil in the old anime, so it amazed me that PGSM actually did this. The only thing I don't like about Dark Mercury is how she was turned; she was kidnapped and forced into evil. It would have been so much better had she been tempted into evil instead by the abandonment of the crew instead of just being physically vulnerable. But I love her water sword and the implication that regular Mercury was just holding back her power.
"Akumi" was okay, but was it me, or did she really not seem all that evil? I was expecting her to be like Mio the way everyone was talking her up.
I love the idea of Princess Mercury and I wish there was more of an exploration of her princess form. However, we don't see much of it.
I'm not as much of a fan of the Super Sailor Mercury or the Eternal Sailor Mercury looks. The classic look is just so iconic that it should have been kept.