With Rek'sai, I have only ever used her as a jungler, but she is able to be used in the top lane role. She has very strong ganking ability and excellent sustain between camps as she uses her burrow.
I generally start with Cinderhulk and then build into Black Cleaver, but she does make a pretty potent Warrior enchant champion for her upgraded machete. The beauty of Rek'sai is that once you build your full jungle item and one offensive item, you can start building tanky with items like Dead Man's Plate, Randuin's, and Spirit Visage and be an absolute terror in team fights. You may not blow people up, but you will soak up an incredible amount of damage. Add in Merc treads and you will be eating up their CC, as well.
My final build with her tends to be something like: Cinderhulk, Black Cleaver, Merc Treads, Dead Man's Plate, Randuin's Omen, and Spirit Visage. If their team composition is particularly AP heavy, I will switch out Randuin's Omen for Locket of the Iron Solari. I hope this helps.