Juggernaut is HARD, only way to describe the hero. If you don't lift you probably shouldn't be reading this, go play Phantom Lancer or Magnus, then play with some broken glass and razor blades. This guide is for playing juggernaut as your farm-lane carry. Not some push-support lamer who's ult can only take out 1 support hero. If you can't farm well, that's another reason to stop reading, but this time you should go play Crystal Maiden.
If you want to be a limp plebe you might buy phase boots instead of treads, they do synergize nicely with blade-fury and aid in last hitting for scrubs. If you are a real baller, however, you will buy treads because of their synergy with omnislash. For any first-timers to the swag-club that is Juggernaut players, omnislash, along with the # of slashes described in the skill tooltip, allows for auto-attacks between those slashes. The higher your attack speed, the more attacks you get and the more newbz you destroy.
Mask of madness also plays into this perfectly, don't whine about MoM making you squishy, everyone will be dead by the time your ult is over so it won't matter, now sack up and keep reading.
Start by grabbing the first 6 items in the item build. You should be farm laning or you shouldn't be reading this. Turn your slippers into a Poor Man's Shield at the side shop, then go for your treads. If you REALLY feel like being gay, go ahead and pick up phase boots instead, but I much prefer treads due to better synergy with omnislash.
Now you need to start working toward your battlefury. This item was made for juggernaut. Not only is it great for farming and general DPS (Go read some articles on how BFury synergizes with crits if you don't understand) but it is also amazing with omnislash. If you don't have a bfury your enemies can just cluster when you ult and split the damage and no one will die. If you have a bfury and high attack speed, you want them to cluster, you will kill them all.
Some fail-tier players will get their Ring of Health and then get the 2 swords for battlefury, this is bad and they should feel bad. Have the courier bring you your perseverance first, this will let you spam your blade-fury more in lane. You should aim to have your treads and battlefury by 15 minutes. If you don't have them by then you are either a bad farmer or have a tough lane. You shouldn't really leave lane at all during these first 15 unless your team needs help stopping a push, you are playing the carry, your job is to farm. Now head on to mid-game.
As soon as you finish your battlefury it is time to really farm your *** off. Grab a quelling blade, post-bfury quelling blade is great. Start autoing creeps now, but make sure you get the last hits. With your battlefury and qblade you can kill waves quickly and send a few of your own to hurt the enemy tower. In between waves, clear a few jungle camps near you. You need to get your mask of madness as fast as possible.
This build really shines as soon as you have your MoM. Spam that thing all the time, it is okay to leave your lane-area now. Clear your whole jungle and both ancient camps, rape lane-waves whenever you cross their path. Never stop moving. If you are farming well you should have your MoM and Scepter before 25 mins, if you get some kills you should have them around 22 mins.
THIS IS IMPORTANT!*!*!*!*!: As you are running around farming like this, your ult should NEVER be on cooldown, as soon as it comes off cooldown, make sure your farming path crosses an enemy and rape their face off. ALWAYS MoM before you ult, this will give you twice as many hits between slashes and no one should be able to live through it, especially once you have your scepter.
If there is ever an open tower as you are running around, kill it, this will help you and your team, don't be afraid to kill t1's even with no creeps around. If there are no enemy heroes that's fair game, just drop your ward and pop your MoM, it will melt.
Now after 25 mins it's time to head into late game.
After 25 mins you should be looking to push, if you don't items up through Scepter by now then try to have your team buy you a little time to get them. You really need your 4 core items before you start team-fighting (Treads, Bfury, MoM, Aghs). At this point you don't really want to blow your ult on 1 person unless it is the 1 or 2 slot on the enemy team, ideally you would like 2 heroes close together when you have the first 4 items, you should be able to kill them both if they don't split.
Your ult lasts a LONG time at level 16 with a scepter, so don't be afraid to play initiator for your team, just MoM and ult, then spin away after your ult is over if you have to. You aim to have your butterfly sometime between 30-35 minutes. Once you have that you can search for 3-4 heroes clumped, you have a good chance at killing 3 of them with this build. The strategy here is pretty simple, teamfight whenever your ult is up and take a tower afterwards. If you aren't blowing your ults on nothing there shouldn't be enough resistance at the end of one to defend a tower.
At this point your 6th item is really situational. I put up abyssal because it's my personal favorite, but really any big damage item works fine. Mjollnir is cool due to shooting lightning while ulting, but with a MoM your attack speed is nearly maxed anyways so the AS from Mjoll won't help you much. I would go for Abyssal, Daedalus, or MKB if the enemy has dodgers. When you have a full inv you should almost treat your ultimate like a black hole from enigma, don't use it unless you can get AT LEAST 2 heroes good with it.
By now the game should be over, you have officially gone hard.