It depends on your playstyle and what champions you enjoy. Speaking for myself, I typically stick with the following champions:
is a safe early pick capable of bullying many opponents in lane, laning against ranged opponents, and scaling well later into the game.
is a strong lane bully against melee champions, great at kiting and chasing opponents, and has massive teamfight utility in mega-form.
is great for shutting down ranged opponents and for dominating the mid-game (though I haven't used her since preseason started). She's essentially a really tanky assassin capable of dueling many opponents.
is a safe lane bully who transitions into a strong skirmisher and teamfighter.
is a strong utility tank who is capable of locking down targets and protecting his team. Even if the nerf to his ultimate's cooldown, it's still extremely powerful and Grasp of the Undying combined with his passive provide a lot of survivability in teamfights.
is a strong duelist and splitpusher who excels at killing (and enabling his team to kill) a tank on the enemy team. I haven't played him yet this pre-season and I am still figuring out what I want to build on him (after the BoRK and Tiamat nerfs), but he should still be a good pick, if susceptible to being kited.
I would recommend determining which champions you enjoy playing and which are suitable for your team compositions or synergize well with your allies' champions.