I had been experimenting with Reza a whole lot lately, and wanted to check out other people's guides and builds for him. I was appalled when I looked at the few top rated guides and realized that they all included Aftershock, Tension Bow, and often Stormcrown. That build is my least favorite in the game for several reasons: It's lazy, easily countered, lacks consistent and reliable damage, and is exclusively mid-game focused. You have no idea how many times I see it built by terrible wannabe Takas. They buy those three items and then suddenly stop building offensive and start tanking up as if they were playing Reim or Krul instead of a bursty assassin. *sigh*
Yes, I know that Reza and Taka are two different heroes, but in my experience, that build doesn't consistently work on any hero, and an inconsistent build is a huge disservice to you and your team.
This guide is for people who want a unique build fit for a unique hero. I believe Reza was designed to work with CP (I mean, it's kind of obvious, considering the fact that all his abilities only have crystal ratios), and this guide is a short summary of my experimentation to that effect.
I always feel like shorter guides are better, because no one really wants to read a novel on how to play a videogame character. Also, I personally just want an idea of what to build and the basics of playing a hero so I can use that knowledge as a foundation to go out and find what works best for me. I hope for you to go do the same :)

Enjoy!

So, I'm glad to see that you've continued to read my guide despite the slamming of most people's favorite Taka build. You've passed the first test: not getting triggered by someone else's opinion on the internet. Well done. Let's get started.
As Reza you have the potential for some of the most powerful basic attacks out of any of the heroes, maybe barring a souped-up crit Gwen.
Reza utilizes his Firestarter heroic perk to deal extra damage with his basic attacks after applying firestarter with his A ability or Ult. This is an extremely fun effect, and the best part about is is the fact that it's not your only source of damage. Your Scorcher deals decent damage, has good projectile speed, and is a fairly spammable ability after overdriving it and building an aftershock. Your Troublemaker is also fairly powerful, as movement abilities go. First, note the fact that it has two charges. This is important when it comes to using your combo more than once. It deals a bit of CP damage when passing through enemies and then empowers your next basic attack, which can stack with your firestarter empowered attack. This is Reza's bread-and-butter combo: Scorcher to apply firestarter, Troublemaker, then basic attack with your aftershock bonus and watch as you take a huge chunk out of your opponent's health. Your ultimate should mainly be used as a gap closer to finish off your target (preferably the enemy carry), or to dodge powerful incoming attacks like Ringo's Hellfire Brew or Adagio's Verse of Judgement.
This build should be primarily used against team comps that have a squishy laner and jungler: really any time a carry is picked for both the lane and the jungle. I would not recommend this build against any jungler, as they are all far tankier than the standard carry.
Next, I'll cover how to play against balanced team-comps that include tankier heroes.

I’ve discovered that there are two main ways that you can play Reza. One of them is as a burst assasin, and the other way is as an ult-driven powerhouse. Lately I tend to favor the latter, because the recent buffs to Reza’s kit makes his Netherform Detonator extremely powerful. At level 3 you have an 8 second netherform, which is a fairly long time to consistantly apply and consume Firestarter. For those of you who don’t know, when you use your ult you catch on fire, grow horns, and apply firestarter with your basic attacks if there is no firestarter currently on your target. This syncs incredibly well with an Alternating Current, because you are constantly dealing chunks of damage. However, an alt current on Reza is really weak early game because you either don't have your ult or you haven't leveled it very much. This is why I recommend dragon’s eye first. The assassin build I mentioned earlier really falls off late game because Reza can’t keep up with other damage dealers during late game teamfights. This is why I prefer the second build, because your ult can cary you through the late game when your single empowered basic attacks can’t.
In the late game teamfights you want to poke at the enemy with your A to keep firestarter applied and to ramp up your dragon’s eye. As soon as you have a good opportunity, use your ult, focusing on the enemy carry. Activate your Shiversteel while you are waiting to reappear so that your basic attacks will slow down your target right away. Try to wait until you’ve landed your A on some of your enemies to ult, because landing netherform detonator consumes firestarter for extra damage and then reaplies it.
Your Clockwork is excellent for lowering all your cooldowns including your ultimate, and consistently being able to land your scorcher means that your netherform is up incredibly often: somewhere around 15-20 seconds.
Like I said earlier, this build is still great against squishes, but can also hold it's own when faced with a tankier opponent.

I still need to play Reza a bit more to find the heroes he's good and bad against, so stay tuned for another update.
Thanks for reading!

~Zerroc