I have been playing AD Nidalee top a lot lately, I usually get 1-2 Dorans and rush Triforce. I'm not sure what should I get after Triforce though to max my damage output. Should I build like an ADC or more like an Assassin or Bruiser? Is Hydra a good item on her?
For max damage? Build Like an ADC without def. Buy 2 Dorans->IE->Berserkers->Trinity Force->Blade of the ruined King->Last Whisper->Bloodthirster.
This would be most damage. However never build max damage on Nidalee!!.
If you are in a real game and you want to play safe and successful you should go for Dorans Blade (1 or 2 depending on when you go back) -> Blade -> Boots (either Mercury or Ninja Tabi) -> Trinity Force -> Randuins/Spirit Visage -> Randuins/Banshees/Spirit Visage -> Situational Last item.
Just buy the item that helps against their main damage source as a defensive item. When it comes to the last item, there could be really almost any item that's viable on a bruiser. You can go for More defense with Warmogs or Frozen Heart, you can go for More offense with Maw of malmortius or last whisper/Black Cleaver and you can also go for a safe guardian angel. For the trinity force part: you can also replace Triforce with Iceborn gauntlet. It does a nice amount of damage and it gives you more armor and CDR.
So yeah. Max damage: ADC, Normal Build: Bruiser. Just keep in mind that bruiser builds are extremely situational.
[This was written as of patch 4.14]
I found EC Skyyart's AD Nidalee build more interesting than the usual Trinity Force, because it focuses on Cooldown Reduction (CDR) to increase her sustained damage, mostly as cougar. The logic behind this is that when Riot reworked Nidalee's kit, they significantly reduced most of her AP ratios. A 40% CDR based build increases her pounce mobility, allows her to throw spears more often (for Hunted-buff-tagging purposes mostly), more heals and traps (AD CDR build increases sustained healing a lot), and of course the most damage is done by her Takedown ability, which deals magic damage increased by 33% on hunted targets, and increased again up to 250% based on how low the enemy health is.
The build goes as follows, with 7.5% in flat CDR runes :
2x Dorans (Cheap stats, needed as Nida needs AD and HP early on to avoid getting killed in a burst)
Sheen (Insane harass in human form while casting spells, added dmg on Takedown)
Bilgewater cutlass (Increased sustain, her heal is mana-expensive)
Iceborn Gauntlet (Replaces Trinity Torce (TF), built-in AoE slow, CDR, mana, some AP, the armor is nice because you don't have armor on TF)
Ionian Boots of Lucidity (Depends on your CDR runes, if you are CDR-maxed (40%) with runes and other items, swap then for mercurys or berserkers)
Blade of the Ruined King (You can complete it earlier, depends on the game)
Banshees Veil (You should already be almost CDR-capped, if you're not you can go for a Spirit Visage)
The rest is up to you, I like to throw some heavily defensive items like Guardian angel and Thornmail to make a very nice fighter Nidalee, with capped cooldowns, she has everything you need. The tankiness will allow her to dish out more damage through the fight, because she will have time to heal it back, when your ADC Nidalee will get bursted without even getting close for those cougar kills.
If you are doing well, you can use a Bloodthirster or another AD-heavy item. I would recommend against both % and flat armor pen, because of Nidalee's hybrid damage (With this build, your main magic damage source is your Takedown, when bursting down a carry).