This build is just a default that I most often use, but can be changed depend on your own team composition and/or the enemy team's composition.

I have earned a 10-win streak on Ranked Matches with this build, so I believe it is somewhat good. Of course, you need decent skills, the build does not make the player.

Armor PenGreater Mark of Desolation/Magic Pen - Red (Armor for early game harass, Magic for late game damage)

Mana Regen/Level - Yellow (Useful to constantly heal or harass with spear)

AP/Level - Blue (Better for late game damage, can be replaced by mana regen)

Health - Quints (Less squishy)

Masteries are just a default mage mastery setup, although be aware your damage output will be more hybrid, which is good to make their tanks buy both Magic Resistance and Armor.

The order of the items can be changed, depending and how you get fed, and if you're feeding. Doran's Ring to start off, good for your mana regen and lane durability.

Boots are just good in general to poke when in cougar.

Now, you want to get the Guinsoo's Rageblade really soon to keep these stacks up, giving you a 90 AP bonus, but if you cannot afford it, get the Sheen before. It'll permit you to harass your lane enemies with your combo - - - , also with a mana bonus that can help.

Mercury's Treads aren't a hurry, depends if opposing team has many mages or a lot of CC.

If you get focused in team fights, get the Guardian Angel after that.

If you're not being focused and need more damage output, get the Rabadon's Deathcap, will make your deadly.

Lich Bane does a really good job when combined with your Takedown and also to push towers. You use your heal buff and your destroy towers very quickly.

Alright, so the core items to a good AP Nidalee would definitely have to be Guinsoo's Rageblade and Lich Bane.

Variable items:
Mercury's Treads, Guardian Angel, Rabadon's Deathcap.

Theses items are not static, you will have to adapt depending on your own team and the enemy team.

If you're facing a Heavy AD Team, you will want to replace those Mercury Treads with some Ninja Tabi, and it works well with your Cougar Form which gives you a dodge bonus aswell.

If you're not being focused and need to help your squishies get some more survivability, maybe grab a Rylai's Crystal Scepter or Zhonya's Hourglass , two decent AP items so that your is more effective.

If your team already has a decent damage output, you can always replace that Rabadon's Deathcap with some other AP items that give you more survivability. Those can be Rylai's Crystal Scepter, Rod of Ages, Zhonya's Hourglass or even a Hextech Gunblade/ Will of the Ancients. Because spell vamp can become effective as you have many spammable skills.

Now, you may have noticed that this build really just makes you buy 5 items, and you have an extra slot, why? Well usually you don't get to the point where you have a max build because it's a quite expensive build. If you're a great farmer or the game seems just endless, you'll get to that point where you need a 6th item.

First of all, you want to buy all elixirs before, because it's always a good thing to have, and also make sure you ward very often, map control is super important.

You still have extra gold? If you're on VERY late game and it seems to never end, I'd recommend some either EXTREMELY tanky items or some more AP Bonus. I recommend these two, because usually you want to end the game by killing the towers. Nidalee is a GREAT pusher, and so getting her more AP will destroy a tower even quicker, heal her for more, and reinforce the Lich Bane's effect. Although, if the opposing team is turtling, you might want to get more TANKY items (dependingly on the opposing team) so that you don't die in a matter of seconds.

Bushwack first, you want to prevent ganks and know where the enemy's at, you want more map awareness. It also deals an interesting amount of damage for it's first rank. Useful to escape ganks once you get the pounce from cougar form.

should be your main skill on your early game laning as it gives you mass durability and good lane pushing if you ever make your enemy go B.

is leveled alternatively with Primal Surge depending on how easy your laning phase is going. If you're having an easy time, or you're on a 2v1 for example, you'll want to level up Javelin Toss to zone the enemy out of EXP range and/or to finish them off. Skillshot skill, so use it wisely because it's very costy on mana.

Flash: This is a very important summoner spell because Nidalee is very likely to get focused. Use this for a quick and easy escape, and Primal Surge to jump back into the fight. Also, if you fail your JUKING* with your pounce, having flash as a backup is always good, as it is very hard to land your pounce through walls.

Ghost: This is a summoner spell that many people use but I'm starting to doubt it's use. It is useful when you are pushing a lane and you get chased, and also to escape in general, but it is not necessary. Becomes useless if you get slowed by the enemy.

Cleanse: I like to replace my Ghost with Cleanse because Nidalee is very vulnerable to CC. You already get a good movement speed boost by being in the brushes, so replacing Ghost with Cleanse can be a good idea when facing a heavy CC team.

Clairvoyance: This skill is almost mandatory on Ranked Games, so if your support doesn't get it, and no one else, you should get it. Nidalee is a somewhat support champ so it's appropriate for her to get this spell. Also can be useful to Clairvoyance the turret when an enemy is hugging to land your Javelin on them.

*JUKING: Juking is when you use skills that makes your champion move to move through walls, and so making your enemy unable to follow you in their chase.

Nidalee can be a jungler from the beginning, but I do not recommend it because she is a very good laner. I never personally tried it, but I have seen it, she's not necessarily a good ganker, so not exactly worth it.

Once you reach level 6, you earn your Aspect Of The Cougar and that permits you to jungle much more easily. With Cougar Form, it means you have 3 Burst Damage skills and your heal. Bushwack can also be useful to deal a bit of damage, but especially lower the creep's armor.

Once you reach level 11, you can pretty much solo Dragon. I do not know if you can solo it earlier, but I mostly do it at that level. You Bushwack the dragon, and then spam your cougar abilities, and / once in a while.

Nidalee is mainly a team player because of her incredible heal. Some will say that her Primal Surge is even imbalanced, which can be true to a certain point.

When in team fights, burst your cougar form, but always be ready to return to Human Form to heal your carries. This is especially efficient if you have an AD Carry because not only will you save them, but increase their Attack Speed. So if they have some Lifesteal, they'll regenerate much faster, and that can decide whether you win a teamfight or not.
Also, important to know how to teamfight. First of all, when no one has yet engaged, just poke them with the javelin, it deals incredible damage and has wide range. It can take half health of those glass cannons.

Farming with Nidalee becomes an easy task once you reach level 6. Your - combo does a considerable amount of a damage, can clear a wave very quickly, which can also be used to push a lane.

Pushing is definitely one of Nidalee's most important role in a game. Although you do want her in teamfights, she makes an excellent pusher.

Reasons:
Self-heal: Able to tank a bit of the tower damage.
Attack Speed Bonus: Both her and Guinsoo's Rageblade stack together perfectly for some good attack speed.
High AP: As you know, a mage deals 50% his AP on every tower hit, and Nidalee gets a pretty high AP with this build.
: This burst damage Attack Damage skill DOES in fact affect towers. So using this when you have a Lich Bane or Sheen will be very effective to "take down" towers.
Easy Escape: If the enemy team goes back to defend, you have a Cleanse or Ghost, and a Flash. You also have your Aspect Of The Cougar(speed) and (speed/juke) to escape, and your Prowl (speed in bush).
Map Control: You can place traps along the lane you're pushing to prevent ganks, so that you're aware if the enemy is coming towards you.