Am a go with Ed on this one.
Junas (Psyren) vs Aizen (Bleach)
Manga reading list: Bleach, Naruto, Katekyo Hitman Reborn, One Piece, Code Breaker, Gamaran, Kuroko No Basket, Fairy Tail, The World God Only Knows, Kimi No Iru Machi, Good Ending, Medaka Box, Nisekoi, D.Gray-Man, Ao No Exorcist, Freezing, Magi - Labyrinth of Magic, Soul Eater, Rosario Vampire Season II, Iris Zero, Ubell Blatt, Bloody Cross, Sun-Ken Rock, and Infinite Stratos,
aizen is just pure pawnage he wins
gray (fairy tale) VS black star( soul eater )
mihawk would need just one shot to kill him.
hiko seijuro(rurouni kenshin) vs zoro(one piece)
hiko seijuro no Q about it
(and man no fan boy stuff please every one knows tha kenshin is arguably the best swords men in the manga world)
Cell (DBZ) VS rob lucci (one piece)
cell is bether
cloud ( finale fantasy)vs haru (rave master)
Haru because the ten commandments own
Dante (Devil May Cry) vs. Train (Black Cat)
Train i think he would have the upper hand
hei (darker than black) VS spike (cowboy bebop)
anime characters and the only anime protagonists whom i dont hate.but i dont think that characters in rurouni kenshin manga are so strong as in one piece for example kenshin could never split apart a huge ship with a swing of his sword the characters from this manga are not so strong.but even if they were you could not call him the best swordsman cause hiko is stronger than him as the writer of rurouni kenshin has mentioned.
Train Heartnet(black cat)vs jirou(black blood brothers)