Sion specifically for me works really well in the bot lane with his Q slow/Knockup and his E slow, his R as well. His CC combo if paired with an adc like Jhin that can snare and cait who can also snare leads into a deadly combo
Sion Combo: E, Q
Cait: W Auto Q Auto
Jhin: W Q Auto
Sion: W R
Jhin: Auto, E ahead, auto, auto (R if necessary)
Cait: E, auto, (R if necessary)
In that order, an enemy adc would easily be helpless against a Sion Cait or Jhin and his R is also very good for roaming to mid lane as well as a good escape if something goes wrong.
To carry the lane a bit harder taking Black Cleaver or Mallet helps to zone off the enemy with your damage, and if you REALLY want to carry, you can build Lethality Sion which can oneshot adc’s mid to late game with his R Q combo and intiates fights very well.
Personally I prefer building him like a traditional tank support and playing around my team, sometimes we’ll totally feed the enemy and still pull a win simply because of his cc capability.
Lethality Sion I would only suggest if you have a very tanky team comp, the damage would bode very well in teamfights.
Tank Sion also scales very well with a Warmongs late game, I usually end with around 5000 hp and I can tank the enemy for days with the mr and armor I have which usually ends up allowing my team to get an ace or an inhib.