Life orb is a very good choice because you get to choose any move and you get x1.3 damage with no drawbacks. However, I prefer Choice Specs because it gives you a x1.5 boost, which allows you to get a much higher damage output. It's mostly up to preference, with Life Orb being the more popular choice, I prefer Specs. Actually, you could also run Scarf to boost its speed tier into the 140 range, allowing it to outspeed many things, including most legendaries due to the majority of legendaries having only mediocre speed. Whatever item you choose, I hope it serves you well in the battle tree.