Sedgar -> General (for maximum stats use bows never lances it helps)
Wolf -> Warrior (for max stats always use axes not bows!)
Ceadea -> Mage/Sword-master (only depending on who you don't have/X Merric/Navvere
The above two are highly advocated reclassing choices. However, they are quite specific and don't cover the majority of units.
A good general tip (used especially in Wi-Fi) is to train a mage to 20/10 (lvl 10 promoted), then to reclass to Swordmaster. Equip the Levin Sword (use an Arms Scroll if necessary) and the character should do hefty dmg (note that the Levin Sword's damage IS indeed based on magic). Also note that you should level up the 10 remaining levels as Swordsman to raise Speed. This tip is good online, but not on the main game since you probably will run out of Levin Swords quickly.
Two tips:
Reclassing DOES change your character's stat growth. When leveling up, two factors determine the growth:
Your character. For example, Shiida has great speed growth regardless of class. She will probably max out speed by the end of the game no matter how she is reclassed.
Class. Some classes add to or detract from a character's stat growth. A reclass to Myrmidon/Swordmaster would boost the speed growth of a character that otherwise has low growth in this stat.
In general, don't reclass a character with poor stat gains in a particular stat into a class that depends on that particular stat. While they will gain better growth in that particular stat, they will not usually gain enough of a boost to perform well as that class.
