I personally bring Ultra/Secret Rares, holos and Reverse Holo Rares and Trainers. (I use a large ring binder for this.) Ideally, regular trainers would be a good idea, too, but I needed to draw the line somewhere. Reverse Common/Uncommons (aside from Trainers) aren't THAT sought after, but you could maybe cherry-pick more meta ones (say, Alolan Vulpix or Trubbish). They're also, realistically, not typically worth much, so aren't especially great trade bait unless you're looking for equally inexpensive cards.
Worth noting that I bring these to regular league as well, not just events.
Keep in mind that, in general, you won't necessarily get a large volume of trades. It's really more about having something for everyone to maximize your chances at a successful trade when you do find someone willing. And to that end, the more you bring, the better. But it comes down to what you're comfortable with, and how much it's worth to you. (And what you're even looking for. You're not going to trade that Hyper Rare Zoroark for that reverse holo Gloom to complete your Burning Shadows set, now are you?)
