I am going to talk a little bit about my favorite Special Wall in NU: Altaria!
Altaria has an excellent Base 105 SpD stat with a good typing. This allows it to be a good special wall with resistances to Grass, Fire, and Water-type moves, as well as an immunity to Ground-type moves. Its Stealth Rock weakness is mitigated by its reliable recovery in Roost, and Natural Cure lets it shake off any poison or sleep inflicted upon it when it switches out, making it much more difficult to cripple (especially since I play Altaria by switching it in and out repeatedly). It also has access to other support options, namely Heal Bell. In addition, it gets Toxic, Roar, and Perish Song. Here is my favorite set:
Altaria (F) @ Leftovers
Trait: Natural Cure
EVs: 252 HP/252 SpD/4 Spe
Nature: Calm
IVs: 0/31/31/31/31/31
-Dragon Pulse
-Heal Bell
-Roost
-Fire Blast/Ice Beam
This set maintains good coverage while supporting the team very well. Dragon Pulse is the main STAB and does decent damage, with exceptional neutral coverage. Heal Bell rids the team of paralysis, sleep, and poison--this is helpful because most NU clerics are Grass-types, so if your main sweeper is a Ludicolo under rain, having a non-Grass type to heal it of paralysis will be helpful. Roost lets it do its job repeatedly and minimizes the concern of the Stealth Rock weakness. Finally, a coverage move is preferred (in my case) in the last slot. Either move handles Tangela well. Fire Blast lets Altaria get through Metang, while Ice Beam handles opposing Flying-types and Torterra/Golurk.
You may prefer Perish Song, Earthquake, or another support move in the last slot. Oftentimes I use Altaria on Sun/Rain teams, replacing Leftovers with the appropriate rock and the last slot with Sunny Day/Rain Dance. In particular Altaria synergizes very well defensively with common Rain sweepers due to its resistance to Grass-type moves and immunity to Ground-type moves.