Welll.
Tobi- Pros: you get to be funny or 'like tobi' the whole day. Also, costume is relatively easy.
Cons: You'd have to make the mask and find out how to get the overcoat thing he has. If you wanted to make it, it'd take some work.
http://i176.photobucket.com/albums/w195/naruto666_04/Tobi_Akatsuki_Madara_Pein_364_by_st.jpg < just a reference.
Or, you could do THIS Tobi, which is much less popular, but easier.
Hidan: Pros: His hair would be EASY for you to get. Just flip it and die it. Also, he's really cool. And not many people cosplay as him.
Cons: If you wanted to make his weapon along with him, it'd be a lot of work. And like I said before, you'd have to find someway to make the akatsuki jacket.
http://tn3-1.deviantart.com/fs11/300W/i/2006/177/f/9/Hidan_by_pokefreak.jpg < reference
Eureka: Pros: Almost no one cosplays her. You'd be a sight that most people recognize, but it would only be you instead of herding of others cosplaying the same thing. Also, her hair would be relatively easy as well, just die it blue and change around the hairstyle.
Cons: She has blue hair, which could potentially turn your hair blue-ish if you used some kind of dyes (a wig would solve this) and some people MAY not recognize her, although I imagine most should. And her character is really quiet. And a pretty hard costume.
http://www.absoluteanime.com/eureka_seven/eureka.jpgTalho: Pros: She's awesome, and the costume looks cool. Even if people didn't recognize your character, they would still think that you're awesome.
Cons: It's a very revealing costume, and if you're not the kinda person into that, then this is out. Also, the costume may be harder then the rest to make, merely because it used little amounts of cloth in weird ways and is not from a highly popular show, so there is no costume you can just buy and wear.
http://www.fujicreation.com/eureka7/talho01.jpgEh...that was long! XD
