There is no definite answer to what their stats are at 99 because Espers have bonuses that are applied whenever the character levels up. Esper X may give a +5 to strength while Esper Y may give +5 to magic.

All of the stats are actually pretty useful, so I can't really say which is the most important. It depends on what the character does. Strength won't mean anything for a character that solely uses magic. Same with magic to a tank.

Personally I like to have a high speed stat because in a way it can sort of double your attack, along with other things. If you're fast enough you can attack twice in the time it may take another character to attack once!