Malphite is not a true tank, he is technically a fighter. However, he is best built as a tank. Malphite's passive gives him 10% of his health as a shield that recovers every 8 second delay in which he is not attacked, benefitting from damage resistances. But Malphite's natural health/mana regen and health/mana are very low, making him very easy to wear down and kill when his bubble isn't up. Additionally, Malphite's abilities do not synthesize well when he is built as an ap or a physical damage character. For these reasons, Malphite is best when he is built as a tough fighter with high armor. You will rarely be capable of sustaining a high damage output with Malphite, but his ap abilities do naturally high base damage and his ground slam gets 70% armor as added aoe damage, so late game Malphite can burst enemies easy during a fight.
This guide goes through how to build Malphite's items properly, rune choices, and skill sequence.
Always check the enemy team before picking your first item. If they have ignite, always start with a ruby crystal; if they do high ap damage take a nullifier mantle and 2 health potions. A cloth armor/2 health against high phys teams without ignite. Doran's items are ok, but the 3 choices i mentioned allow you to start your other gear sooner.
Start by rushing Heart of Gold or Boots. Heart of gold is a good starting item for Malphite because it is built into Randuin's later and provides health/armor and gold early. For boots I recommend Merc treads because they make it easier to escape disables and provide magic resist, which is usually low on Malphite. Ninja tabi is usually only better against a few auto-attackers and even then the Ionian boots might be a better pick. (Boots = First complete item)
Save up for a chainmail vest, if the other team has a lot of ap spend the money on a negatron cloak first so you don't have to worry about ap damage until much later in the game.
After this point, this is the build I recommend for most games, depending on the other team, you might need to build health or a banshees or a force of nature. If you do not need them continue as follows.
After the chainmail, get a mana crystal and build a glacial shroud, Cooldown reduction is very for both helping allies escape and helping them get kills.
After the Glacial shroud, build armor, you usually want 2 chainmail/cloth and one of the other. Build Frozen heart and warden's mail. (Second complete item)
If the other team is doing high damage to you with ap, build your negatron cloak into a force of nature or a banshees, both are equally good in my oppinion. Force of Nature is better against damage by %health and high ap, banshees is better against high ap in short bursts and ignite.
Else if the other team isn't killing you with ap damage yet finish randuin's omen.(Third Item)
Same choice as above here, but if they aren't killing you with magic damage build a thornmail first. If the other team doesn't do magic damage based on health and seem to have high enough magic resist, I recommend building a warmog's instead of banshees or force of nature, yea it's late game, but warmog's is good for you anyway. Health and regen. (Fourth and Fifth Item Choice)
For your sixth and final item, the choice is yours and entirely depends on the other team. I usually play twisted treeline where there is a lot of ap on both teams, so the other team has magic resist and low armor. I know i recommended manamune which is a really weird item to get late, especially on malph, but if the game is really long and you need physical damage, it is one of the best choices. With frozen heart and possibly banshee's, you already have high mana and you get more with manamune. So it does a lot of physical damage. Other items are listed below
Zhonya's Hourglass: high damage item for malphite.
Sunfire Cape: good sustained damage, something malphite lacks and health. Helps alot but I usually don't get it because i roam in and out of fights when i can, keeping my slow up and bursting whenever i can. Works good for chasing because attacking melee while chasing with malphite is an easy way to let enemies get away.
Second thornmail: if you like burst damage and don't like waiting for it, or the other team does high physical damage, this is for you. Once you get over 400 armor in a game, physical damage even with last whisper does very little to you. This gives you 70 extra damage to ground slam at a cheap price and allows you to tank a little better. And why not slam for 550-750 damage once in a while.
Second Frozen Heart or Omen: same principle as thornmail, just extra regen/health or mana in exchange for a higher cost and slightly lower armor boost.
Second/Third Magic resist item: you usually won't need this.
Warmog's: late again but it helps you survive.
Atma's Impaler(if you have warmog's) or Frozen Mallet.
Sorry if this section was hard for you to read.
For skills, you always start with shard unless you are foresting, it pokes enemies at range and does moderate damage to a single target as well as stealing speed so you can chase or escape better.
Even though you start with shard, start on ground slam at level 2 and max it out as fast as you can. Always max it out at level 9, it makes it harder for champions/monsters/minions to kill you and your allies and makes armor better and less necessary at the same time. Don't worry about brutal strikes yet, just get your ult at level 6, and shard every level you can't get ground slam and you'll be fine. Brutal strikes is a waste of mana when you only use it to buff ground slam for 1 use or to block a small amount of physical damage. Early game it is unnecessary.
After ground slam is leveled, work on shard as fast as you can except for getting ult rank 2 at level 11. After shard your only choice is brutal strikes and unstoppable force.
I know most people level shard first and i know the reasonning behind it, but trust me there are not enough reasons to take shard. Ground slam is aoe and easily does more damage to every single target. Slam is less accurate but you don't have to see a target you can kill stealthies or bush jukers with it. Ground slam weakens minions and auto-attacks so your allies can escape more easily or win a group fight, and last but not least, your ground slam is the most effective thing you can do to weaken enemies going for your tower besides and sometimes alongside your ult. Which means you can turtle and turn a losing game around into a win alot better with Malphite than almost any other champion in the game.
Personally, I had a game where I was 0/5 early game with both outer turrets down and ended the game 8/6 with a victory where I and an urgot carried.
Ground slam is Malphite's primary killing and support ability, so learn to use it right and level it first.
Until a recent patch, Malphite always had terrible magic resist - still does bad when you have to build too much of it. Taking Magic resist glyphs is the best solution to this. Take mr because it still gives quite a bit of mr early game when it isn't as important and starts giving you more mr than flat runes at level 12 when you start needing it most. I also recommend taking one scaling magic resist glyph to help push your magic resist up.
Armor per level seals work well because they begin to pass the flat ones at level 12 and they provide an extra punch to your ground slam. It sounds pathetic, but if don't have enough armor against ap teams you will still die. You need armor for damage anyway, so why not get another item's worth from runes? Take a scaling armor quintessence for added effect.
For marks most people use spell pen on malphite so they can actually kill, it isn't much of a difference late game so i still take extra mr. But honestly you should take whatever you think you'll need here. Health/damage/ap/armorpen/magicpen/resistances/regen/cdreduction if they have it. Your pick.
For quintessances, I already recommended 2 resistance quintessance once. You can take a third resistance, or a health or ignore my advice and take whatever 3 you need. (Spell pen is common).
For malphite I recommend Clarity for when you are low on mana and ghost to help escape and chase people down. Different people have different tastes in summoner spells and you should take whatever you are better with. But Malphite still has legitimate mana issues. That don't start to dissipate without getting at least 400 extra mana (glacial shroud) or mana regen, and even then clarity comes in handy. Malphite is one of the most useless champions in the game when he is low mana, so Clarity is one of the best insurance policies during a fight.
Exhaust is good as always.
Cleanse is ok for escaping, you could just take the mr cap if you want that though and get a better version.
Ignite is just cheap, it does a ton of damage past resistances(not my complaint that part is justified, my complaint is the high damage) and occasionally does more than it says. Benefitting occasionally from the offensive tree. (Supposed to do 410 at level 18, really does as much as 456 in some cases).
Clairvoyance/Fortify/Rally/Revive are all generally considered bad choices for a reason. though you can take them if you're really good (or bad for Fortify).
Suprisingly all three trees can work, it depends on what you want most. For me it is survivability, so i take 21 points in defense, taking mostly the damage reducing stuff considering ardor is not good on malph, and the remaining 9 in utility for clarity, ghost, reduce dead time and xp boost.
Offensive is good if you think you survive well enough. Malphite does a lot of spell damage so the ap, cd reduction, spell pen, and bonus damage are nice. I still recommend 6 points minimum in defense for early game armor and magic resist.
Utility is good for cd reduction-a must on malph, lower cooldowns on summoner spells-another good thing, and other bonuses.
The magic resist calculator is slightly off compared to the in game one, not sure why. In game this build and item choice gives me 199 magic resist at level 18.