This is the build I use, as well as Hotshot's build. Hotshot is considered one of the best players in the game, and the number one Nidalee. This guide will help you become an insane Nidalee.

9x Greater Mark of Desolation
9x Greater Seal of Evasion
9x Greater Glyph of Focus
3x Greater Quintessence of Desolation

The armor penetration marks and seals are a definite on Nidalee. They will allow you to ravage your enemies in cougar form. The dodge seals are very useful, because with the masteries in dodge, and Nidalee's cougar form, you are able to benefit greatly from the dodge. With cooldown reduction glyphs, you can maximize your cougar DPS, and also get the most out of your heal.

The run 0/9/21 masteries because I feel it is the most useful. The 21 into utility is very nice because of all of the cooldown reduction, speed, and other useful things. The 9 in defense is really nice because of the extra dodge and armor. These masteries are by far the best.

The items I have picked are by far the best for Nidalee. They allow you to dominate. With the items I have chosen, they allow you to do a huge amount of DPS, and your heal will also heal for an insane amount late game. I will write a short definition on them here.
Doran's Ring : Very useful because of the Health, and with the AP your heal is much more useful.
Mercury Treads : No other choice for Nidalee. The magic resist is extremely useful, and since most of your dps is in cougar form and you are fairly squishy, the 35% CC reduction is extremely nice.
Gunisoo's Rageblade : This is Nidalee's best item. The AP/AD maximizes her DPS, and it's very easy to keep the stacks up with her W in cougar form.
Lich Bane : I love this item. It has everything you could ask for. AP, movement speed, magic resist, mana, and of course the sheen effect.
Rabadon's Deathcap : I don't think I even need to explain why this is good. Once you buy it, you will see a huge increase in damage/healing.
Banshee's Vail : By now, you will be pretty damn squishy. Everything about banshee's is very good for you, and it's just something you NEED to get if the game lasts this long.
Void Staff : It's pretty unlikely for a game to still be going on before you get this, but if it does, I recommend getting a void staff. The extra magic pen will help counter anything the other team has bought.

For the skill sequence, you ALWAYS want to max heal first. It is extremely nice early game. You also only need one point in your trap for vision/protection. It's pretty much an easy skill sequence. Just max heal, spear, then trap.

For summoner spells you always want to take ghost and flash. Nidalee is pretty squishy, and you will regret not getting them. There really isn't anything else that's useful. I've seen ignite used before, but it's really not needed at all.

Farming on Nidalee is VERY easy. Once you have cougar form, you can just pounce onto minions and spam e+q. Of course, playing as Nidalee, you should be farming as much as you can while you aren't doing anything else.

Nidalee can be very useful in team fights. Try to heal your teammates on cooldown, or yourself.

One unique thing I find on Nidalee is her spear. If you see some of the enemy team just derping around, feel free to try to hit them with a long range spear to do some nice damage. Also, remember to keep your rageblade stacks up!

Pros:
- Good Damage
- Great Heals
- Great awareness with traps
- Good get away
- Decent Harasser

Cons:
- Squishy
- Bad on mana throughout whole game, unless you have blue. Doesn't really matter too much though, all of your DPS is in cougar.

To summarize this, I am just going to say that Nidalee is in my opinion one of the best champions in the game, as well as the most fun, because of all of her different abilities.
Feel free to comment on my guide and let me know if you have any questions. Good luck!