Just do it! (My Diamond Series - 1/6 so far!)

Nike should ideally be played aggressively, and having finally gotten her to diamond rank a couple of days ago, I thought it best to begin my guide writing with my sixth, but not last diamond ranked god.
Nike is not a god that requires any starting items in my opinion, though this is generally because I have never bothered to buy them with her. Early game she can be used for a menacing damage dealer, and her disarm will benefit the team massively in the early laning phase for Jousts, Conquests and Sieges.
I will do two alternative builds for her, and explain the best ways I have found to counter specific gods.
Enjoy! :)

To begin, Nike's who desire a head start in damage should spend two points on Rend and one on Plan of Actin. Once level four arrives, Valiant Leap will be purchased. In the early laning phase, Valiant Leap is not vital, and I had found it is not needed. After this, Rend and Plan of Action followed by Sentinel of Zeus should be your priority. Valiant Leap can be used as a good initiate, though following the builds given it will be used as more of an escape tactic, rather than an initiate.
Getting Blink as a relic will make her Valiant Leap purely an escape feature, which is brilliant, meaning if a fight turns sour, she can always escape!

For Nike to be effective, she must be played aggressively and used to initiate team fights with her ultimate. Due to being a warrior, she can withstand a fair amount of damage, while returning the same amounts.
For Build One, the Semi-Tank
With this build, Nike can afford to be very aggressive and remain in the fight for long durations of time. Having full cooldown with her means she is able to use Plan of Action at least twice within a fight, securing a kill with Rend or using an accurate Valiant Leap to escape. This build works well with a team that does not have a tank, and requires a survivable figure within the team. Good pairings with this build (for jousts) include the likes of Anubis and Awilix.
For Build Two, Damage
This build (depending on Physical > Magical/Magical > Physical) is best used when there is a Guardian on the team, or another high sustain character in play. Nike's one defense item should be enough to keep her in fights for long durations, but she will need to retreat far more often with Valiant Leap than in the first build. Buying Hastend Fatalis (Physical > Magical) enables her to use her slow with her ult to its full potential, and often enables her to secure a kill or two with it. Again, full cooldown enable her Rend and Plan of Action to be used at least twice in team fights. A good pairing for this build would be Bacchus and Awilix.

As with every god, they all have their week points. Nike has one main on that I have encountered.
...Awilix
Awilix on your team is fantastic. Landing with Valiant Leap gives a knock up Awilix can use her ultimate with, but playing against an Awilix will render your ability useless if not build correctly (and a particularly good Awilix happens to be on their team...!). Nike must be careful when using Valiant Leap as an initiate with an opponent Awilix!
Nike must also be aware of silences (from Hades etc) as this will remove her from her Rend, and could possibly cost you Plan of Action too! To combat this, I highly recommend maxing her cooldown, as I have done in every build.
Nike is also very weak against Ares unless built very Guardian like. Her Valiant Leap will be stopped, and if she cannot get away quick enough due to being slowed, there is a likelihood she will become the opposing teams focus.