The last slot is really very nit-picky. No point in arguing over it now when we still have more pressing issues.
So, we're definitely running Abomasnow, Mamoswine and Landorus then.
I'll go ahead and nominate Walrein since it is a Hail team after all.
Thick Fat or Ice Body
252 HP / 4 Def / 252 Sp. Def
~ Scald / Blizzard
~ Protect / Roar
Its poor defensive typing is definitely made up by its 110/90/90 defenses and Thick Fat resistances OR healing 12.5% per turn. Great synergy with Landorus too.