Flamberge. Stunlocks the best out of all the greatswords. Hammers are pretty underrated too, but they also have amazing combos. As for spells, fire whip is actually great. It's pretty much a slower combustion that can hit twice. Chaos fire whip destroys guys using no poise. 3 hit combo itself, and I'm pretty sure something like chaos fire whip - katana R1 is possible against no poise builds.
In what way is the flamberge better at stunlocking? Lol.
All have the one handed stunlock, none have two handed stunlock, and claymore has the best combo sets.