Tats are really cool, but I'd be careful about what you pick, and where you get it.
They are permenant and even laser removal does not completely remove them from your body.
And if you get one in a noticable place, it could be hard to get jobs, as many companies still forbid tats showing. [Ankle should be fine though]
I'd go with the crecent moon though
I want to get the 'Greatest Hits' flower on my lower back, but I'm still thinking about it.
From what I've heard they are painful, esp in certain places, so as I've stated, and Brandon has, think it over. Don't make a decision you'll regret.