Best Tank+Attacker?

Attrox shouldn't be built tank. Mass amounts of AD, lifesteal, and preferably AS when available.

Zed shouldn't either. Again, mass amount of AD and armor pen.

RekSai works as a tank, but does a lot better as a fighter with some resistances and health on the side. Stacking a Devourer and using a Hydra lets you use the other three slots as tank items to good effect, however, but you're putting off midgame damage for late game teamfight advantages so it's sorta risky, but worth it if you know her well enough to do something like that.

Fiddle is glass cannon or gtfo. The only "tank" you should build on him is a Zhonya's Hourglass or an Abyssal Scepter.

Voli can safely build a Devourer/BotRK depending on if you're jungle or top and then go straight tank and have a really nice damage output. People underestimate how much he can hurt once he gets his Frenzy fully stacked, and with one on hit item he can actually win most 1v1s provided he isn't kited endlessly.

Nautilus needs buffs. He can't even tank, let alone try to splash in offensive potential..

Gnar builds wonky. Most tank items aren't that great on him, since most tank items don't do much to help his mini form and that's what you spend 95% of a match in. If you're building a "tanky" Gnar it's through health items, not necessarily tank items, which gives him a bit more survivability without really adding in any resistances.

Shaco should be built AP, AD, hybrid, or not at all. Like Fiddlesticks, tanking should be the last thing you think about doing with a kit like Shaco's.

Out of the ones you listed, I would suggest either Rek'Sai or Volibear.

Volibear's damage output, like I said before, can be surprisingly high with only one on hit item, and his passive lets him get through the early game while still retaining his function as a tank by using it wisely, baiting out opponents who don't realize that your passive is available and will keep you alive long enough to take them down. The longer the match goes, the more tanky you get, while your one offensive item is still pulling more than its weight combined with his attack speed. Popping the ult just gives him even more dueling potential, and makes him a terror in teamfights. He's actually one of my favorite junglers because he's a sleeper anti-carry. No one really expects a Volibear to be able to rip someone to shreds, but just one on hit item and you're pretty much guaranteed to. He has the sticking power through his Q and his E. If you're top you get the extra sticking power from the BotRK active, and if you're jungle you can build a Stalker's Blade instead of Skirmisher's for the Chilling Smite to achieve the same purpose. If a match is going obscenely well, take a Wit's End for your last item, pop your ult in a teamfight, and watch the enemies drop like flies. The only downsides to Voli are that your ganks are a bit weak, you rely on enemies being massively overextended early on because the MS boost on his Q isn't that noticeable, his slow is pretty meh until it's fully ranked, and he lacks hard CC even though that flip can really hurt if you know how to use it.

Rek'Sai has a lot of utility through her unique vision, and easy ganks. However, from the very beginning it's been suggested not to play her as a straight tank, but rather as somewhat of an offtank. Like a slightly more offensive Vi, basically. You want her to start with that Devourer's and get it stacking, and after you get either a BotRK if you're worried about tanks or a Hydra if you're going to be focusing on farming and tearing into squishies, you can pretty safely build a tank item or two, and depending on how the match goes build a third as well. But damage is a bit more important on Rek'Sai than tank, provided that you know how to use her well. Not to say that tanky Rek'Sais don't work, simply that it isn't as outright effective overall.

Purely on the fact that Voli requires a VERY innate feel for his kit to use as an attacker instead of just a big, furry ball of "ult harder, I have an itch," I'd advise Rek'Sai. Her ganks are a ton easier, and she definitely does attacking better since she has built in AoE on her Q, her knockup can hit multiple targets, and she has a lot more mobility options.

Rek'Sai is the best fit for high sustained damage/tanky fighting...

Gnar is great too; he does good single target damage in little form (even if you build mostly tank contrary to what the above said). He has good area of effect burst in mega form too, and a whallop of crowd control on top of that.

The rest on your list are more tank oriented or squishy carries.

Another good pick for your description would be Xin Zhao. He's a straightforward and powerful fighter and ganker.

And lastly I recommend Jarvan too. He has great ganks. The way Jarvan is played though is usually as tank in the jungle. When played top lane he can afford to build a more lethal amount of AD though that let's him burst squishy targets. He can add tank items after that.

Tank: Nautilus > Voli > Rek'Sai > Gnar Fighter (attacker): Voli > Rek'Sai > Gnar > Nautilus > Attrox Assassin: Zed > Shacco > Fiddle

The only true tanks you listed are Naut, Gnar and Voli. Rek'sai is best built as a fighter, as is Aatrox The rest are assassins.

edit* if you want a tank with insane kill potential, gnar and voli are your best bets (moreso gnar)

gnar s literally a tank + attacker

Tank + Attacker is what people call bruiser. Zed is certainly not a bruiser...

gnar easily