I actually think the best mixed wall to be Cofagrigus. It is just quite simply an amazing Pokemon. The typing gives it two key immunities. Add to that the only having two weaknesses, both of which are not as common as other attacking types. Now then, onto stats. This thing has 145 base defense, which is quite simply amazing. Base 105 Special defense is also quite nice. The movepool it has is perfect for walling. Will-o-wisp is an obvious move, crippling physical attackers while doing damage. Equally obvious is Night Shade, which will do a fixed 100 damage everytime, which is good when considering one of the next possible move paths. There are two choices for the final two moves, each equally useful.
Trick Room and Disable. The strategy is quite interesting. Set up Trick Room. Wait for the opponent to use an attack you want them to not use. The next turn disable that attack (since you will almost always attack first thanks to low speed). Not only is the foe out an attack, but you've also stalled a turn for Leftovers healing and Will-o-wisp damage.
Protect and Power Split. By having low IVs in Attack and Special Attack plus a Special Attack negative nature, you can easily bring down any offense the foe may have. It won't matter to you since Night Shade is a fixed damage attack. Protect is like Disable in that it stalls a turn for Will-o-Wisp damage and Leftovers healing.
Cofagrigus is in my opinion the best wall of this generation. I even think it superior to Dusclops (if only by a tiny bit.)