Juggernaut is quite a deceptive name for this nimble fighter. His basic stats lean towards a more aggresive role - the second lowest base attack time in the game allows him to get more critical strike procs and generate aggression. Because of this, I find Juggernaut best paired with an aggressive support, such as Shadow Shaman or Lion. He is extremely versatile and can build virtually any item that you would see on a right click hero, but his aforementioned low BAT generally means that you want at least one source of high AS.
Otherwise, Juggernaut's Omnislash allows him to get kills even with zero net worth, as its damage is constant, making Juggernaut hard to keep back in terms of farm, especially when he chooses to build a Battle Fury.
As one of my favourite heroes, and honestly one of the easiest, let's get into it.
Juggernaut's first ability, Blade Fury, makes him spin around extremely fast, dealing high damage to those he can keep on top of. While in this form, Juggernaut is spell immune, essentially providing him with a free Black King Bar in terms of escape or avoiding nukes. Generally you want to use this skill for early kill potential, as later into the game it is simply not worth being effectively disarmed for 5 seconds when your right-clicks deal more damage. Perhaps if you built a Monkey King Bar or Abyssal Blade you would want to reconsider when you use this ability, as these items have effects which can proc during Blade Fury
Overall an amazing skill that scales well and is useful for farming stacks and securing early kills, but does not fit in with a right-click heavy Juggernaut far into the late game.
Healing Ward is a skill that prevents Juggernaut from being a self-centred right-click hero like Slark or Alchemist, who have little way to help their team not die in a fight. Not only useful in terms of fighting however, this ability allows Juggernaut to stay in lane for much longer than other melee carries with little sustain. Due to this, you should be able to spend much less gold on healing items and be able to focus more on farm. Situationally, you may want to put more points into this ability before Blade Fury if you're getting bullied out of lane by aggressive heroes such as Pudge and Zeus.
Juggernauts 2nd skill allows him to keep his team alive where other carries would be unable to, and sustain himself to farm up his core items all the quicker.
Juggernaut's Blade Dance is what allows Juggernaut to be played as a core. It has an amazing value point, and upgrading it later only makes you more deadly. Being able to keep enemies in place with items such as Diffusal Blade and Abyssal Blade allows you to get more attacks in, which means more crits, which means more damage.
I tend to find this ability incredibly frustrating when going for last hits, as occasionally you may get a crit which throws off your entire lane equilibrium, makes you 30 gold short of your Divine Rapier at the end of the game when the enemy is pushing your Ancient, ultimately making you lose the game.
Seriously though, this ability is amazing, and even with the significance of its recent nerf it is still amazing at shredding entire waves of creeps and enemy heroes in a few brief swings.
Omnislash is what gives Juggernaut his comeback potential. Basically a 600 damage nuke at level 1, which keeps you on top of your enemies, it allows Juggernaut to be much more aggresive. I tend to solo lane until level 6 at which point I Blade Fury on top of the enemies and Omnislash when they try to escape behind their creep wave, taking advantage of the sheer power of this ability.
Further points into this ability simply make it more useful in team fights, as with a maximum of 17 slashes if you purchase Aghanim's Scepter], it allows Juggernaut to avoid most sources of crowd control while you absolutely destroy the enemy team. If you have high enough AS, you can also attack while this ability is going on, dealing up to twice the damage of the base skill, or more if you have the damage to do so. The skill goes through Spell Immunity, allowing you to pre-emptively jump on an enemy after they use their [[Black King Bar, a tell-tale sign that they are planning to fight soon, and dodge whatever it is they throw at you.
Without doubt the coolest of Juggernaut's abilities, Omnislash gives Juggernaut the means to kill supports in a team before going for the main guy.
Juggernaut should almost always go into the top lane, paired with an aggressive support who allows him to keep the enemy within range for Blade Fury. Never be afraid to go for last hits, as Juggernaut needs as much farm as he can take to get an early Battle Fury, and his Healing Ward allows you to have constant sustain. Level 6 is a major power spike for you, as it is for most heroes, which allows you to go for and turn around kills. Be aware of your lane situation and if you are getting harassed too hard, usually by ranged heroes or spam casters, you can always go into the jungle, or look for kills in other lanes with your ultimate.