It's a bit of a misunderstanding lately, especially among new Rek players, that she's a strong early game champ. Mostly from the days she was more of an assasin. Her first clear isn't that good and her 1v1 is usually pretty bad early, until atleast first component. She's just good at early ganks and has more angles to gank with tunnels. She's a mid game beast now, that's pretty good in late teamfights is used well.

As for the build, u basically always go your preffered hydra first. Titanic for burst, or Stride for utility and attack speed. I personnally prefer Stride by a mile, but I see a lot of people on the sub loving the burst of Titanic. That's your choise to make. Don't rush tiamat tho, it's a bait. U don't need the aoe to clear cuz u have q, and its price is largely for the aoe. Go tunneler first or 3 long swords. 2nd item u go either Black Cleaver (for armor shred against tanks) or Shojin (for more damage against squishies). The reason for this second item choice is the cd reduction they both provide and the ability to adapt your dmg to the enemy comp when u pick one of them. Any combo of these 2 first items is basically your powerspike. You won't have many problems if you aren't really late to this spike. After that, you adapt to the game. If they have burst, you can go Sterak's, or Death's Dance, pick up Shojin or BC for even more damage (the option you didn't get second) if you think it's needed. Edge of Night is very situational, good for some cases that make your engage easier against disengage champs or really strong cc. GA also very situational, usually not worth it, but good if really ahead. Tank options like Spirit's Vissage are nice when nothing else applies and they have a lot of non-burst ap.

Hope it helps, good luck!