Akali is an assassin and as such to a degree, naturally countered by tanks since their designated role is to peel her off their carries (she can be quite hard to peel though, especially if she get's rolling), she also snowballs nicely given you manage to get ahead.
She does have pretty powerful sustain in her kit though, so I think she could probably do well against tanks that don't rack of a load of sustain, in lane.
Graves is strong all around and as a marksman his role leads him to become a natural counter to tanks, he's very tanky himself and with items built out of vampire scepter can pretty much outduel any tank at most stages of the game thanks to the sustain he gets. I think he's more succeptable to assassins, at least if he hasn't ramped up his "Grit", fighters and mages.
Lissandra has lockdown potential and cc that can rival pretty much any tank, she's also pretty mobile. Being AP she can force alot of tanks that really want to itemize in armor early into more uncomfortable build paths. Not sure how well she does against tanks that have strong sustain and like to build MR early though, like Maoki.
Riven is more or less countered by the tank class, being a diving fighter they are the natural counter to her class (fighter),, but being one of if not the most versatile champion in the game, if you're good enough with her you can outplay just about anyone and anything, skill ceiling and floor alike are very high and she's incredibly easy to mess up with, she can beat pretty much any tank and get beaten by pretty much any tank, it mostly comes down to your skill and your opponents understanding of how her kit works. She might be somewhat on the weak side atm though.
Swain is a nightmare to deal with for many tanks since they are generally quite immobile and succeptable to his bind. Like Lissandra he can force alot of tanks into uncomfortable build paths. He pretty much outsustains any tank in the game but not until lvl 6 when he unlocks his ult. Regardless, a skilled Swain can make any tank's lane a living nightmare. I'm no Swain main but I'd assume his core weaknessess are being quite immobile himself, succeptable to ganks, squishy before lvl 6 and not quite possessing the brute and reliable initiation that hailmarks proper tanks although being having soaking abilities that can rival tanks.
So I guess it all comes down to you, tanks are aqually a little on the weaker side at the moment (since they get the least out of masteries compared to the other champion classes) so their natural counters (mages and adc's) fare pretty well against them, fighters and assassins don't quite fare as well as these are the classes tanks naturally counter but that doesn't mean you can't beat a tank with them, especially in 1v1 combat since tanks are extremely team reliant, especially later in the game when their scaling starts regressing.
Ergo, take whatever champ/playstyle attracts you the most and learn the matchups, like not picking Jax/Tryndamere into Malphite etc unless you are very proficient at that particular matchup or a one-trick pony.
Pretty much all champions have at least 1 champ from just about every class in the game that either naturally counters them or can do well against them, thus it's not very likely that you will be beating every single tank in the game with one single champion.
My 2 cents.