most of the top bans are actually assassins right now.
Evelynn, Yasuo, Rengar, Kha Zix.
Top ban is Kassadin whom plays like an assassin once fed.
The only "must bans" for me are Amumu and Kassadin. It seems like I *always* lose against Amumu. I hate that guy. Kassadin is just annoying and since most games go late in Solo-Q then normally you always have to deal with a fed Kass, which sucks.
I never ban riven. Riven players can be exploited. They all play the same hyper-aggressive way. I pick into Riven with Tryndamere most of the time and just farm at my turret, bait them to dive me, pop ult and kill them. If they can't get baited then as long as I don't feed I will be able to outscale them. Riven can't run from a Tryndamere. Ghost+Mocking Shout+Spinning Slash+Cutlass Active is plenty of utility to keep the gap closed. Tryndamere usually loses to Riven until you can finish at least Stattik Shiv+Cutlass, unless you've fed Riven then you will always have a very hard time. Just need to avoid the all-ins pre-6, most Riven players will jump your shit at level 1 for a quick first blood. It's so predictable though so can easily be avoided. Be especially careful if they 1st buy Fort Pot.
Most players wouldnt pick Tryn into Riven, I even accept the fact that Riven technically is a counter to Tryn, but I have played the match up so damn often that I'm really comfortable with it and will win more often than not. I also realize that not every Riven player is good and if I ever faced a good Riven I would probably get my ass handed to me almost every time. Luckily most players are actually super cocky and don't use their brains when playing Riven.