Senna is in rough shape right now, mostly due to a lack of quality mythic choices. Duskblade, Eclipse, and Kraken Slayer have all been popular Senna items before getting nerfed. Losing one or two mythics could maybe be withstood, but all of the small nerfs to these items add up. As of 13.3, Senna's last solid first item, Umbral Glaive, also received a nerf.

Even without a direct nerf, her power has been chopped away at by gradual item nerfs. Not to say that Senna can't still be an effective pick, but it requires more work now than it has before. Luckily, it seems like 13.4 will bring Senna buffs, so we'll see where those changes leave us. But for now, Senna is not a great pick in most scenarios. Unless you're counterpicking, she's not a generically strong option right now. Fingers crossed for 13.4.

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I think it depends a lot on what role you're playing in your team. If you're filling an enchanter role, for example, it's not really on you to itemize against tanks. For most of the AD builds, however, you get the option to counter with your item picks.

As for what I prefer to build, it depends. Kraken Slayer gives a powerful option if the tanks you are fighting are building a lot of armor, but I don't like it as much against tanks that mostly stack health (Sion, Cho'Gath, etc). It's still a powerful item, especially with Senna's bonus AD, but I find it to be a bit less effective on average. Eclipse does better against health tanks, but I honestly find the most effective item to be Lord Dominik's Regards if tanks are a big problem in a game. It handles both types of tanks pretty easily and still gives me flexibility in my mythic slot. And, of course, you can stack it with a tank-killing mythic item like Kraken Slayer if you are desperate for that kind of effect.

TL;DR: probably Kraken Slayer, but don't discount Lord Dominik's Regards.

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I'd typically take Ultimate Hunter, as it gives you more presence on the map post-6. With how long of a cooldown Senna's ult is, it's nice to have it more often -- gives you more choices.

Treasure hunter isn't bad, but Senna doesn't really struggle with income. If you're confident in your laning phase and roaming, Treasure Hunter will help subsidize your weak mid game.

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