This is my first guide, so bare with me. To start off, this build was actually something I recently decided to experiment with because of two other guides I read on here. Many people like to play him tanky AD, but Malphite isn't really the champ that's going to get really close auto attack you to death. That would be the job of your AD carry. Also, hes not a pure tank. His ult is good CC, but after that he can only make one person slow at a time, so once you use that ult, you're not going to be able to keep their team off your squish teammates all that easily. I play Malphite as a super tanky anti-carry.

Okay, here is the reasoning behind my rune selection:
Spell Penetration marks : good for all of your abilities except your "w" spell.
Using these runes helps alot for early game laneing when harassing with your "q" and using your ult.

Defense Seals : Obviously you are an off tank AND your passive makes your shield better with the more health you have

Magic Resist Glyphs : Once again, you are an off tank...self explanatory.

Movement Speed Quints : I like the extra movement speed because it stacks well the speed gained from your "q" and I don't build boots first.

I like going with 0/21/9 because of the sustainability it gives you. 9/21/0 could work too, but malphite is dependent on using his abilities to do anything, so I like the extra mana from the utility tree.

I start off with the basic solo top start, Regrowth Pendant plus a potion. Some people like getting a health potion off start, but malphite has enough sustain to where he doesn't really need it. Mana potion is much better because it allows you to harass much more early game, which makes it easier to farm. I go straight to buying a Philosopher's Stone for the extra money and regens. After that I buy my boots. Most of the time, I buy Ninja Tabi because it gives armor, which makes your "e" spell deal more damage, but Merc Treads are also a good choice if the other team is AP heavy. After that I build a Chain Vest and Negatron Cloak . The order i buy them in depends who I am laneing against. Then I build a Glacial Shroud to raise your armor a little and give you extra mana and cooldown reduction, which are both very useful. Then I build another Chain Vest for more sustain and for later use. Next I buy a Blasting Wand, which I then turn into a Abyssal Scepter. This is for AP (q spell and ult) and magic resist, because every good team you play against SHOULD have some AP champs that can deal some damage. Then I build a Giants Belt for more health and bettering your shield. Then I grab a Guardian Angel for armor, magic resist, and the revive. Some of my friends said they would rather build something else, but I like it because it gives good armor and magic resist, and that revive can come in handy BIG time. If you are low on health and a team fight breaks out, you need to be there. This gives you the ability to ult right into the middle of their team so your team can pile on the damage, and still get out after they "kill" you. Doing that will most likely result in a few champ kills for your team, and you will still get out alive (probably). Next I buy Rylai"s Crystal Scepter. I used to build a Rod of Ages, but I found the Crystal Scepter a little more useful. The AP is nice for your abilities, the extra health stacks with your shield, AND because you aren't a pure tank, this helps you get a little closer by adding some more CC to you "e" spell. Now you have an AoE slow because of Rylai's :). Then I finish off my core build a Frozen Heart. This is so crucial to my playing style its not even funny. I think it is one of those items that were just meant for Malphite. The armor stacks with your "e" spell, extra mana is appreciated, and it lowers enemy attack speeds by 20%! That plus the cooldown reduction PLUS your "e" spell equals very unhappy AD carries on the enemy team. Once you charge in with your ult, make sure their AD carry is in range of your "e" spell and use it immediately and keep spamming it on their AD carries. They have 70% less attack speed when you do that which lowers their damage output tremendously. 1v1 with Tryndamere just got easy. The 6th item on the final build is completely situational. Some good choices would be: Warmogs, Thorn Mail, Banshees Veil, Force of Nature, Sunfire Cape, plus there are some more. It depends on what you need to make you better against their team.

Once again this is my first guide I have made on here, so thanks for baring with me on reading it xD. If you have any feedback on if you liked it, didnt like it, or what I should try adding to the build, please comment. I'm always looking for a better way to build my champ to make him as best as I can.