I've played a bit of Rek'sai top, simply because I love the champ design but hate jungling. Skill max I go Q > E > W, putting a point in R whenever possible. Depending on what you want/what your team needs, you can go for a more tanky or a more bruiser build.
Tank:
12 - 18 - 0, with CoC as the keystone. Rune-wise, flat AR yellows, scaling MR blues, flat AS reds, scaling CDR Quints
and
or 
This build gives you a nice mix of MS, split-push potential, Health and resists. The only thing it really skimps on is LS. Your main focus will be to knockup targets in teamfights, Q - Hydra - E to burst. Your overall damage will be subpar, but with runes/items you'll have 25% CDR lategame, which put your abilities on pretty decent cooldowns.
Bruiser/Onhit:
18 - 0 - 12, with Warlords/Fervor as the keystone. Rune-wise, flat AR yellows, scaling MR blues, flat AD reds, flat AS Quints
and
or 
Your first item will be Rageblade, as it synergies with your Q pretty well. Your ult gives you quite a bit of hidden power, so in extended trades you'll be doing a lot of damage. Most people won't expect your damage output to be so high, so its pretty easy to bait them into an easy kill for you, especially if you have Fervor. You also splitpush extremely well, and have a good amount of sustain, especially if you take Warlords. In teamfights, you can isolate and shutdown targets pretty well. Deadman's gives you a nice slow on your first auto, and you can normally up your Rageblade pretty quickly, if you can land your knockup. Since you actually have a bit of AP with this build, your burrowed Q actually deals some damage, and can help you CS from range, if your in a particularly bad lane.