Champs to play with
1. Miss Fortune is amazing with malphite. You ult, then she does then you exhaust the enemy adc, and Q their support, as well as activating your W and auto their adc and use your E when you can. It is likely that their adc will die, so make sure you focus the enemy support after the enemy adc gets low. This way, your slow will come up sooner and be in range to try for that double kill
2. Twitch is also great. Because of his high burst damage and attack speed, once you ult, the fight it over. Seriously, ping twitch to go under his Q before you ult them, so twitch comes out of his stealth with higher attack speed and a ton of damage. He should also drop his E for a further slow, making a double kill almost inevitable.
3. Lee Sin is good at helping creating Malphite's ult. Lee can Q in, then ult someone back. This is the person you exhaust, then, while the enemy team is focusing Lee, ult the rest of the team. 3 of your team members should follow you while one (If lee ulted the adc as he should, this should be the AP mid laner such as Ahri) focused the exhausted one
4. Obviously Yi. With your ult and his ult/Q combo, you will dominate. Hands down
5. Ashe is viable here. Her passive makes sure that once you ult someone and ashe autos them, they will have a hard time getting away.
How to play Malphite
Play passively heading into 6. If you are lucky and get a gank, just make sure to Q down enemy champs so they lose their speed and therefore their getaway. Getting Sunfire first is awesome, because it gives some armor (50) which is buffed by his W's passive, as well as giving him 500 health, which is necessary for tanks. By 30-40 mins, you should have 2500+ health as well as 300+ armor, making you unstoppable against AD teams, just make sure to keep your Targon's brace until the late game to have enough gold, if all your kills and assists wont give you enough already
Your ultimate is, in my opinion, top 3 in the game. Landing it is critical for success, and if you don't, you can likely throw the game. Always target the assassin or the adc. Both of these roles usually include fairly mobile champs, so make sure to catch them when their guard is down. One serious threat is Katarina. With her extreme mobility with her daggers, you must catch her after she moves to a dagger, but before she ults. This makes Katarina an ideal ban. Some other assassin threats are people like Talon, because you must catch him before his ult lands and wrecks your team, and Rengar/Kha'zix because you must ult them the SECOND you see their shadow creeping up on someone. With adcs, watch out for Vayne and Twitch. Twitch less so, but mainly because once he comes out of invisibility, you must ult him or he will ult and wreck your carries, but he is only a main threat if he is fed. The biggest threat is Vayne. With Vayne's ult/Q sending her invisible, she is impossible to track. If you don't catch Vayne before she goes invisible, you can rely on one of your carries to lose about 1/2 of their health before you ult them and even then you will likely only be ulting Vayne so other enemies will still be able to fight back. Using Malphite ult takes skill, and knowledge of your opponents so make sure you know any enemies possible escapes.