Refined Siegmund

your best bet is the first which guarantees follow up attacks with 90% or more hp or the second one with the 18 mt which increase his attack stat by 2, as Siegmund has 16 Mt or the one which def +4 to increase his durability

I actually personally consider that his best nature thanks to the weapon upgrade. Just play him carefully. His speed isn't very good even with an optimal nature so he's prone to doubles.