As a Sion

These 3 will give you a bad time. Fiora will stun you with your stun, or just dodge it. Also, %HP true damage is fun. Riven will never let you land a Q on her, there's very little you can do to say 'no' Trundle's pillar interrupts your Q, blocks your Ultimate and he steals your stats.

I'd imagine that Shen is a hard counter, poking with E would probably be the best approach there.

Don't go for extended trades, and play EXTREMELY safe when she hits 6. Never try to ult her unless your jungler is ganking and she's already used her W. Focus on farming.

Try to roam earlier than you otherwise would. You can bring more to your team than you can trying to make something of this obnoxious matchup

What TidalTyrant said basically. Most of this stuff applies to the others; You're better off not trying to ult Riven, she'll probably just dodge it and 100-0 you before you can make it back to your tower, monkey that she is, and Trundle can just block your ult and steal all your tank stats.

just focus on farm, roam early and watch for your jungler.

Your biggest threats are people who know how your kit works. I'm talking about the people who are constantly walking around you while they fight you so that you can't get a good Q on them, the players who pop your W because they know it takes 3 seconds to be poppable for you, the players who when they see you ulting right at them actually move to the side so you can miss entirely. Annoying champions that make these things easier include but are not limited to; Quinn, Fiora, Renekton, Gnar, .

Now for the sustainers. Anyone who can outsustain you is going to beat you early on in trades. Olaf comes to mind, Volibear gets to have an execute, and a passive that will let him live long enough to actually kill you with it. Trundle will literally just farm until 6, and you can't normally do anything about that if he's good at avoiding your damage. As soon as he gets to 6, he will stat check you and straight up win the 1v1. ESPECIALLY if he's good at saving his pillar to stop your Q. Mundo can't really straight up fight you until he gets a decent amount of bulk, but as soon as he's got it, if you and him are equal in CS and levels, he will beat you in a fight. Vlad has been pretty gutted as of recently, but give him an inch and he'll take a mile. You WANT to bully him early before his sustain starts to actually matter, because as soon as it starts mattering, it's already too late.

On to the people who outdamage you. Graves, Ryze, Kled, and after waiting until 6 Jax all outdamage you innately. These are ALL champions you should be maxing E against in an attempt to poke them out, because not only are you never landing Q on these champions, except maybe Jax if he used jump to get on you (and maybe not even then if he just dances around you with the rage passive of Phage), but you shouldn't even be trying to fight these 1v1 monsters. I would have put Irelia in the sustainers paragraph, but honestly she'll just kick your ass WHILE sustaining through the lane. Remember, start Doran's ring against ALL of these champions and just max e to poke them out.

I think that pretty much covers all of the biggest threats to Sion.

anyone with dash/cc.

Anyone with Dash, CC or Sustain and half a brain will poop all over you. I used to play Sion top a lot and it seemed that pretty much every top laner in the meta counters Sion. The only time you can win is when the enemy top lander doesn't understand your kit and eats your Q's or stands around and lets you kill him with your passive.