Items

EARLY GAME ITEMS

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  Tabis gives you extra armor, will help you against (AD) poke, and is probably the best type of boots you can build on AD Bruiser Nautilus, since you get Tenacity on Zephyr, you don't need Mercuries. Lucidity is an option, mainly if you need the cooldown reduction for your shield when you have to fight in 1v1 often or when facing harassing opponents.
  
  
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  Get tiamat soon, it will give you extra life regeneration, attack damage, and excellent passive and active abilities to push your lane and farm easier. If you can prevent your enemy from taking any creep, you already won the lane phase, so keep that in mind.
  
  
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  Hexdrinker gives you extra Magic resist and attack damage, plus a great passive against AP toplaners. It is quite good for early damage. Of course only buy it if you are facing AP.
  Granting you health, attack damage, a chance to slow (extra source of Crowd Control!), I really like Phage. It also works great with attack speed items. May also help you escaping (sometimes).
  
  
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  Warden's mail is great against attack speed-based champions, such as Jax or (AD) Teemo. It's always a good item against any AD toplaner, due to the nice amount of armor it gives.
  Giant's belt will help you getting an early Warmog or Frozen Mallet. Your W (Titan's wrath) stacks with HP, so that's a nice item too.
  If you are in need of mana, cooldown reduction or even armor, Glacial Shroud is an excellent item for you. You can also build it Frozen Heart later.
  
  
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  Consider buying Avarice Blade if the enemy is denying you last hits, or if you are going for Atma's / Youmuu later-on. The crit chance is not that amazing, thus I don't usually buy it often.
  Recurve bow is excellent for 1v1 fights, all you have to do is press W and auto-attack for some damage. It is also good for last hitting with your passive. Only build it if you're going for Wit's end though.
  Stinger is excellent in my opinion. Nautilus needs cooldown reduction, here have some. Nautilus also needs attack speed, well have some more. You can combine it in Zephyr later which is a core item on AD Nautilus.
  
  
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  Pickaxe will grant you some extra damage, but it is there mainly to help you build other items such as Tiamat, Maw of Malmortius, Zephyr.
  Vampiric adds some sustain through lifesteal. Extra attack damage is good also. You can upgrade it in a Ravenous Hydra or Bilgewater Cutlass.
  
  
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  Bilgewater will help you take the advantage in lane fights, with lifesteal and a nice active ability (be careful about its long cooldown though!).
  The Brutalizer is good against champions who are stacking armor, such as Malphite. Cooldown reduction is also great for Nautilus.
  Sheen will provide you with some Ability power (remember that [W] Titan's Wrath's damages are magic), Mana and an excellent Active. Can be upgraded in Trinity force too.

MID GAME AND LATE GAME ITEMS

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  Ravenous Hydra. Not only does it provide 75 attack damage (only Sword of the occult with 14+ stacks gives more damage), but also sustain : life regen and lifesteal... And the best passive and active abilities you could ask for. Cleaning jungle camps, pushing your lane, going in 1v2 has never been so easy. This item will make you - wait for it - Legendary (you still need skill)(batteries not included).
  
  
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  Zephyr is quite good in the way that it grants Tenacity, Attack speed and cooldown reduction. It's a core item on Neutilus, and Riot agrees with me since they added it in the recommended items list. It's overall a nice item that I like to build.