Throwing my two cents.
Gerik- Arguably the best. Good bases, con, and comes with a promotion item to boot.
Dieck- Another good base merc, though his growth rates are a bit Jaigen. Get used to looking at that ten speed for a while.
Echidna- Super biased love for her design aside, she's meh. Also IS are bastards for giving her a steel axe with 9 con.
Oujay- Uhhhh Oujay... Well if you want to raise 'em. There's the islands. Suffers from the same low con as Echidna though.
Raven- Raven is secretly a gambler. Unfortunately the RNG likes to play with fixed dice.
Harken- Why is there no love for Harken? Especially Hard Mode Harken? What you want Karel? Get outta here!