For some reason every time my ranked team comes up against a mordekaiser we never seem to be able to counter him and he ends up wrecking us. What champs/strategies are bests for facing him in TT?

Well I enjoy playing morde and here are a few things that I notice will give him trouble, Garen: he can silence morde very ez and has a shield he can use to get up nice and close then if team focuses morde garen can assasinate him when he gets low take care of him real fast just silence have team jump him with cc then garen ult. Also strong and mobil ad carries will wreck morde some can poke out of his range and when they get IE and PD those crits rly start to lay him out. Also shen is good vs morde shen's W is very spammable and if he ults a teammate you can use stand united and save them preventing ghosts taunt also works just like a silence so it too with a lil focus from other players can help u take down morde.

Take advantage of his not so great early game. He's pretty bad at dueling until midgame or at least until he gets some spell vamp. Champions like Riven, Renekton, Rumble, Lee Sin, and Urgot shouldn't have too much trouble against him unless they're fighting him while he's getting shield from a wave of minions as well as from you. Like, if you try to 1v1 Mordekaiser early game you probably won't have that much trouble.

Get ganks from someone from bot lane (if it's 2 man) or your jungler. If he isn't pushing he probably doesn't have that much shield up to negate damage. If he is, he's making it a bit easier to gank him.

Guardian Angel and QSS remove his Ult.

Don't get baited when he's at level 6 as his ult does have a pretty strong heal (and also gives him a bit of shield).

Get MR (obviously)

And here are a few "lolsy" things you can do (might not be actually helpful but a bit fun):
When you're going to die with his ult on you anyways try to get as far away form your team as you can because the ghost spawns where you died (but if you're out of the natural ghost leash range it spawns where Morde is IIRC)

Mace of Spades' "cast time" depends on how much AS Mordekaiser has (which is why I'm assuming his base AS feels pretty high). If say you bring him down to 0.2 AS (1 attack per 5 seconds) it'll take him like 5 seconds to land that spell (which is pretty lolsy when he's building say Lich Bane Morde and it takes him 5 seconds to land that super-Q which should be ample time to CC him or something to stop him from doing so)

qss

Just wanted to say this is false. Mace of Spades resets the swing timer. Good mordes melee hit then use Mace of Spades for the exta swing.

I play morde often, and the best way to counter him is to shut him down early. He beats people hard because he out levels and out itemizes a passive team. You need a bruizer top that can constantly harass morde when his shield is down. Also anyone with a silence, stun, taunt or knock-up will ruin his day. Any time not spent casting as morde is his downfall. QSS is your friend. It's better to 'waste' a slot on an Item that will save your life, than to build that cookie cutter build and feed.

A silence, ignite (if he has spell vamp and a considerable about of ap), qss, or %hp damage (madreds bloodrazor, or abilities).

Or, just do what I would do and play Poppy.

try to get nukers like ryze and annie, they should take down the shield and damage him, also try to stay out of range when you are not going to attack him and if he push to get more shield gank hard, if he doesnt push much he wont have much shield to cover

Gank

gank

gank

Mord is very weak early against ganks. We run jarvan brand and udyr and we are currently 1730

Silence or stun, ignite or exhaust, zoning, burst and ganks. There are so many ways.
But then again pretty much any champ is countered in the above manners.

Ganking.