I'm some scrub that likes juggernaut. I always underestimated his late game potential until I got to know him, and then I also came to understand that with jugg the game does not need to go late. Juggernaut is the most versatile hero when it comes to carry related items besides maybe Wraith King. I will briefly go over skills/items and how you should change them according to the game and have an in depth rant afterwards.
Now the late healing ward and leaving it to the end is like everything in this guide, SITUATIONAL. If you do not have decent or any support, which is guaranteed in my bracket, you should level it earlier. The sustain of healing ward is more important than the damage of the spin if you're having a rough lane. If a support has stacked some camps then I also get an early ward for me to take it. Ultimate at 6,11 and 16 with crit left a bit before maxing. The reason I like my stats is because you will have mana to spin and use your ultimate at level 6, while focusing on the crit will leave you even more mana starved than you already are. Feel free to swap out any ability at any time because there are times you need healing over the crit but you will rarely, if ever, delay maxing spin because of the insane DPS and reduction in mana cost.
The starting items I never change. The stout shield helps me when I get frisky with melee offlaners and if I spin the moment I get to lane to hopefully buy myself a wave or two of farm with the damage dealt to my offlaners (upgrade to PMS at side shop if needed).
I almost always go aquila before shoes unless I see I have kill potential or if courier is unavailable due to death or mid. After I have phase boots and aquila (do NOT be afraid of picking up a wand if they have spammers) I look at their line up. If they have a spectre/naga/alch/AM, basically anything I will struggle to out-carry or out-farm, I go drums and push towers as often as possible. If they have carries I can bully a bit or farm slow I go for a battlefury however this is very situational. The mjollnir is a mix between the two. You can go drums and still get either of the others or both because they all work great for different reasons I will explain when I go in depth on his mechanics.
I have my phase boots, aquila, a tp and a farming/pushing item. I always get a yasha. You could go MoM, basher, etc but I'm pretty set in my ways when it comes to my yasha. I go manta if they have silence otherwise I'm a hoe for SnY on jugg (slow,damage,armour,movespeed,health,looks sexy). I then go with what the game needs, I will go more in depth with examples later.
Notice I did not mention bkb anywhere, this is because when you get it is different every game. Keep in mind waiting too long might be too late for the high impact you need and getting it too early leaves you starved for damage.
You can farm pretty quick but also get involved in fights early, just remember you are not as tank as you would like to be so early. Ideally I farm jungle camps and push towers but go looking for a kill when ult is up when the laning phase is over. If you are behind try to pick off supports in the back of team fights with ultimate and farm hard. Use healing ward in team fights and try to save your allies with it if you can, the regen is insanely high. Juggernaut is all about balance. You need to remember that when you play.
Some players with twice my hours do not yet know that you deal no damage when you hit people while spinning, you only apply effects of your items. This means that the cleave from battlefury will deal damage to surrounding enemies, slows from SnY/skadi/orb of venom will pro and mjollnir/maelstrom will proc. Daedalus will show crits but the damage will not be dealt to the target.
Some interesting things to take note of:
Now as I said Juggernaut is very versatile and is ok to rush big items on like an MKB. This being said, having an SnY and AC seems to have worked best for me so far. I love getting butterfly and tend to skip on basher/abyssal. If they have desolaters or other armour reducing methods I would say AC is core. If the damage I take is mostly right clicks I go butterfly but if the problem is magical burst IN SPITE of my bkb I would go heart or skadi. I am also a fan of a moon shard as a sixth item as it raises your already high dps through the roof.
I go desolater myself if they have something like a sven or dazzle and I rarely get a daedalus but it is a good item if more damage is needed.
I did not mention any friends or foes as I feel jugg is a very good jack of all trades style hero. He works well with any laning partner that has a stun or slow (honourable mention to rubick as he is my favourite to play when I am the support).
Counters to jugg I would say are generally super tank heroes but as juggernaut says, "for every problem, there is a solution." He is countered by most level 3 carries when late game comes around so try to end early.
I did not mention aghs on jugg. This is because I have never felt the need to get it however I recommend it against very tank opponents such as Slarder, Dragon knight and others.
Juggernaut is a very good carry but does struggle against the traditional hard carries late game. He has very high kill potential early game while having farming capabilities and good pushing power with the correct items. He can fend for himself and join in early fights to hopefully net kills and accelerate farm. He is by far my favourite hero to play and he alone made me realise that I wanted to play carries instead of supports. I promise you with practice you will understand this hero and have a tough time tearing yourself away from him afterwards.