I've found two medium armours that I quite like on my casters:
- Luminous armour is great on clerics. It works especially well if you use spirit guardians against groups of enemies. Every time you deal radiant damage the target and nearby enemies get a stacking -2 to attack rolls. Walking into a group of enemies while spirit guardians is active can easiely cause them to get a -12 or even higher debuff on their attack rolls. So they miss every attack that is not a critical hit.
- Dark Justiciar Half Plate (both versions) grants advantage on con saves. Which is a slightly improved version of the warcaster feat, in my opinion the most powerful feat for casters.
