leon (+def -spd)
summer gaius (+res -hp)
takumi (not worth it)
rebecca (+atk -res)
jeorge (+def -hp)
clarisse
Okay let's compare all of these, factoring in those IV's.
Leon has 34 Atk and 27 Spd
Gaius has 30 Atk and 37 Spd
Takumi has 32 Atk and 30 Spd
Rebecca has 32 Atk and 34 Spd
Jeorge has 32 Atk and 32 Spd
Clarisse has 31 Atk and 34 Spd
Mostly it depends on what set you plan on using, but Leon, Gaius, and Rebecca are the 3 best options. If you want high attack, then go Leon (he
is a great BB user because of his fantastic high attack). If you want to go for quad hits, go Gaius. If you want a balance, go Rebecca. Compared to Rebecca, Takumi has 4 less Spd and the same Atk, Jeorge has 2 less Spd and the same Atk, and Clarisse has 1 less Atk and the same Spd; so those 3 can be easily ignored.
I'd also mention that if you have a +Atk Setsuna lying around somewhere, she has the same Spd as Gaius but with 1 extra point of Atk, and since she appears at 3-4 stars, getting good IV's on her is much easier compared to some of the others so you could go for that as well.
Mine is too. Summoner Support + Life and Death + +3 spd seal + Threaten Spd brings him to 45 spd
That's cool, but you're losing out on some Spd by not using another archer and you aren't gaining any extra Atk for the lost Spd. Plus summoner support is for the most part wasted on offensive units and works much better on defensive ones.