It says useable so Im going to assume you mean the most useful in battle, I know a lot of other people took it that way which is why Cleo was considered.
Suikoden 1: Ruby - Elven Sniper does great long range damage, and that's key for a caster for me being long range without needing a pixie rune, also extremely strong magic and a strong affinity for water magic made him my go to healer and personal fave.
Suikoden 2: Would be zamza if he got a friggin forehead slot, awesome mage with a strong physical attack and decent defense, short range weapon a minus but with the fire sealing rune can do great physical damage in regular battles (you can also add fire lizard rune to his weapon then) but without the third rune slot for a magic rune all this is null and void, you either take him as a fighter or as a mage... That being the case Tengaar - an excellent mage with a solid long range attack and a strong unite with Hix who is also usually a welcome addition to my party in place of the over used Flik.
Suikoden 3: Joker - A very solid mage with strong fire magic, and more or less a monk. When leveled up properly does between 6 and 8 attacks in melee combat and it helps he's a member of my favorite team. I usually buddy him up with Jacques and the two make a solid pair. Straight Mage there are stronger, Piccolo and certainly Estella or Sassarai, but Joker remains the best rounded.
Suikoden 4: Ted is the easy choice, strong long range attack, outstanding magic stats and the Soul Eater... Jeane being playable in this game makes her a fun choice too though
Suikoden 5: Zerase is generally a go to for me, long range, strong magic and a unique rune make her a strong choice. Levi is the classic Mage choice for the game and his magic is awesome, I just hate losing a rune spot to the friggin pixie...