Congratulations...you have discovered what many of us long time Malphite players knew long ago.
Malphite...is NOT A TANK!!!
He has never in his history of existence ever even once been listed as a tank on Riots Website. Not now, not ever. Is he tough...yeah. Does he benefit from stacking defensive items...you bet. Can he lock down the enemy and keep them from melting your squishy teamamtes...???....not really. That last point is where the wheels come off in the Malphite = Tank debate. Sure he has a niffty ult, but even with max CDR its only up every 60 Sec. Compared to other champions who have more reliable and spammable way to lock down enemies, Malphite is sub-optimal as a tank by the above definition most consider it to mean.
Tank = capable of mitigating damage for yourself AND your teammates.
Since he only mainly only mitigates damage for himself, it's a stretch to call him a tank by that definition.
However, there are a few things Malphite does excel at that make him an excellent counter to certain types of enemy compositions, and sort of tanky/tough at th same time...causing a lot of this confusion.
He absolutely will hard counter teams that over commit in the area of auto attacking physical DPS. If an enemy teams only real DPS comes in this variety, a Malphite geared up with his GS attack speed slow, a Frozen Heart aura attack speed slow, and a Randuin Omen's active attack speed slow, (which all stack by the way) will completely gimp any teamfight they attempt against your team. The likel yhood of competing against such a team is pretty rare, but a Malphite speced that way against it is almost auto win.
He also makes for a really good counter to enemy channeled Ults. Enemies like Kat, MF, and Twitch who can put out a tone of DPS with their ults hate patient Malphite players. These Mals, wait for the perfect time to ult into a channelers face and blow them up with his semi-impressive burst damage. If an enemy team is overly reliant on their channeler to win them fights, they will be sunk.
Finally, Malphite has a very nice (though somewhat buggy) initiation ability. Rather than act like the sheild that other "true" tanks do for his team, he acts like the bettering ram. He uses his ult and toughness to bust into a fortified enemy postion, blow everything up, cause some havok, scare the pants off thier squishy, and then wait. If his team is a good one, they will see his opening and strike, causing a near automatic victory. This is why he was so feared in ranked games for so long...his awesome initiation ability. On the other hand, if his team is a bunch of uncooperative nubs, they will watch as the enemy tears him apart brick by brick and rage at you for overextending. This is even worse when his buggy ult seems to miss knocking anyone up even though 4 champs are directly under you as you land your ult. With a well coordinated team he can be very effective, in a pub its hit or miss.
I wont get into any specific builds right now, but a well balanced build that includes Mpen, AR, MR, and a smidge of AP can go a long way to success. Good luck finding one that works for you, and...
Stay Rock Solid,