Core items
-Starter: Most common starter is Selflessness for an instant shield on your teammate near you if they get hard CCd. If they enemy team lacks CC/you have a pressure ADC (Iza for example), you can get warflag to push early leads via pressure in lane.
-Thebes: Best tank item in the game currently for both prots and additional health+mana.
-Stampede: Ymir is a pretty immoble character if he has his wall on cooldown. Stampede allows Ymir to engage/disengage easier with movement speed. This active item also can save teammates or enable them to attack the enemy.
Notes: Third item can be swapped out for a different support active such as team beads, hexstone, shield splitter etc
Items after core/options
Pridwin: Great for cooldown, has decent prots, and gives you an on command shield.
Shoguns: Helps the team clear OBJs/Towers faster and synergizes with Ymir passive autos as well.
Spectral: Great into multiple auto attack comps/crit.
Ankh: Antiheal standard item
Spirit Robe: If you're getting CCd a lot, its a good pickup giving you a lot of prots and healing over 6s after being hard CCd.