On suicidal, AK-47, on anything lower, bullpup.
I require one-hit-kill headshots from my rifles, and the bullpup just can't do that on suicidal. I can kill specimens much, much faster with the AK. Also, sometimes you have no choice but to spray down a husk at close range, and the Kalashnikov does a much better job of that as well. It's simply higher DPS. The AK carries more than enough ammo in a mag for me, and even if it didn't, reloading is ridiculously quick as commando.
Ideally I'll have enough money to ditch the backup rifle and carry SCAR and M32. I've never once had the SCAR run out of ammo before the wave was over.