Juggernaut is a powerful mid game agility hero that can be run as a carry or a semi carry. He performs very well with agressive supports that are able to slow or disable the enemy allowing for Juggernaut to use his bladefury skill to get early kills in lane.
The guide here is how I prefer to play Juggernaut, and how best to play him if your team does not have another hard carry(or hard carry that is farming effectively).
This build requires you to have a fairly easy time farming in the lane, preferably in your safe lane with a dedicated support.
This guide may not work out to be the best if your team already has another carry, or if your lane is under constant duress.
Juggernaut does not scale as well as most other agility heroes into the later part of the game due primarily to low stat gains and specifically a shallow hp and mana pool. His ult is arguably one of the best in the game at neutralizing the enemy heroes, but to perform at its best, his ult must be used when the enemies are unprotected by their creeps.
Bladefury gives Juggernaut a great early lane presence and with decent help makes him a likely threat for a first blood. This skill is at its peak in the early game and should be maxed. It doesn't scale great from lvl 1 to lvl 4 but its advantageous over his other skills.
Omnislash is his ultimate and should be leveled whenever possible (ie at 6,11,16).
From levels 2-10, I prefer leveling his stats or attributes when not leveling his Bladefury or Omnislash. This is again under the assumption of a decent to ideal lane protection where healing ward is not needed.
The additional levels in stats add significantly to both his mana and health pool. Without leveling his attributes and no other additional items adding to his mana pool, up till level 7 at max mana he still does not have enough mana to use both his ultimate and his Bladefury consecutively. Going stats eliminates this problem without the need to spend money on items to buff his mana pool which is not conducive to his carrying ability.
The reason I get attribute over Bladedance is that with such low initial damage (46 at lvl 1), Bladedance statistically does not actually add more damage on average attack, than the additional buff damage that stats provide. Also it is unlikely that the user has a big damage item before level 10 which would gain an advantage from a crit passive. This is why I start to max it at 12, the user should have a damage item and should have scaled enough just from the levels, that they are doing enough damage that a crit passive ability helps there damage output.
Juggernaut's healing ward is a very powerful spell and although I save it for last when leveling Juggernaut's skills, does not mean it is a poor ability. The reason I hold out so long on leveling his ward is more because it is held back by leveling his other skills in the way shown above.
If you are in a constantly harassed lane I would get a level of healing ward at level 2 and another at level 4. This guide however is for a carry Juggernaut, meaning you have lane protection from constant harassment.
If your team is going push heavy exchange a level of Blade Dance at 13/14 and instead get a level into healing ward.
Juggernaut has a very low starting damage level of 46. This is problematic for his early last hitting, and therefore a quelling blade is a borderline must have. There seems to be this general theory that a quelling blade is a noob item for people who can't hit. This is not always the case considering some heroes with poor damage are actually unable to get creep kills under the tower. The tower's output damage is just off that if there are multiple creeps under your tower (especially with your creeps attacking) it is sometimes mathematically impossible to get all 4 of them.
As a carry you should be getting as close to 4 creeps per wave as possible.
The branches and regen items are there to provide overall positive stats and to provide lane sustainability.
When you reach 450 gold, go to the side shop if possible and buy a Blade of attack, if not of course ferry it with the courier. This helps again with Juggernaut's low damage. Buy the other Blade then before the boots to complete your Phase Boots. This is situational on buying the Blades of Attack before the boots. If you are more likely to get hero kills early buy boots first; if you are just going to farm, buy the Blades first.
His core item is a Battlefury. This gives him a good amount of damage and provides a cleave attack, helping him farm faster once acheived and providing cleave damage to multiple heroes on his auto attacks from Omnislash. This will create a huge damage output to all enemy heroes when you ult, making it significantly more powerful. Depending on your lane- purchase the Claymore first if a passive lane and the Perseverance first if less so.
Mjollinir is very powerful on Juggernaut. It is statistically proven to provide the highest dps of any item besides Rapier, if the hero has over 150 damage (which by this point you have). It also seriously increases his attack speed, which gives him more auto attacks when he is ulting. THIS IS IMPORTANT. The main reason his ult is so powerful is because of these auto attacks that occur while he is ulting. The number of auto attacks that occur can be found here. Mjollinir raises his IAS to help induce significantly more auto attacks, and provides yet another aoe damaging ability through lightning procs.
Abyssal Blade does very well on Juggernaut at this point in the game considering he has
Juggernaut is a carry that tapers off late game in comparison to many other carries. Your job as Juggernaut is to initially farm very well, maybe get a kill or two in lane, and start to collect your items.
He is a hero (like many other) dependent on your teams ability to hold decent map control without expecting you to roam to gank much. Juggernaut is very powerful after level 11 with the additional bonus slashes with his ult and the user should try to get him to that level as soon as possible. With additional items his ult becomes very powerful with the potential to severely reduce the enemies numbers or strength in a team fight. Once he has amalgamated his Battlefury and parts of his Mjollinir, Juggernaut needs to go on the offensive and start pressuring the game for big team fights in which he can wreak havoc upon.
Despite his simple abilities he isn't the easiest hero to learn how to maximize, but hopefully this guide can give you some help and allow you to understand his strengths and how to cater to them as well as how to defend his weaknesses.