These are the characters that have the highest MAG growth from each game:
SI - Luc, Crowley, Hellion, Tengaar
SII - Luc
SIII - Luc, Sarah
SIV - Ted, Maxine, Jeane, Warlock
ST - Jeane
SV - Zerase
It's difficult to compare characters between games because variables change. Suikoden I only allows 1 rune, Suikoden III had skills, Suikoden V had modified magic stats, Suikoden Tactics has different growth rates, etc. I personally think that whoever is under the Tenkai Star (Star of Idleness) would be the best mage per game. These are Luc, Ted, and Zerase. They all have the highest magic growth in the game, and in case of someone having the same magic growth as them, they beat them out because they usually have above average speed. Obviously, there's an imbalance because Luc and Ted had True Runes, and Zerase had a powerful unique rune.
That is only speaking from a game-wise perspective.