Especially in QM it is very important to be flexible and adapt your build to what you encounter; this will rarely be some “standard” or “meta” comp. Assumptions about ally/enemy builds and playstyle are also out of the window with that. Additionally, expect to see Aba on the enemy team in 95%+ of games due to matchmaking-rules.
Level 1
The choice is definitely solid here as Portal Mastery and the movement speed are rarely relevant, but sustain is very essential as many games will lack a healer and small damages can add up quickly (also mana).
Level 4
I recommend Dust when going against an Aba that actually places mines (not alway the case), or when stealthers or Zagara are present on the enemy team. The armor can be neat if the enemy has DoTs r other delayed damage and you want to ensure the escapee´s survival. The additional damage is nice if you want to use portals to finish off low-health enemies at the fountain.
While by experience people will use it for escape, do not count on anyone using it offensivly. It does happen, but that is rare.
Level 7
Remember that this requires a damage trigger, so if the use more trickle-damage or do not coordinate well this may fail. Arcane Explosion is pretty nice if you have a melee diver on the team to add additional damage on top of the protection.
Level 10
Polybomb loses a lot of value if it can not bounce, so i would recommend not using it when the enym team is a bunch of ranged assassins only. Better to set them up with Ley or at least take some of them out of the fight for a few seconds.
Level 13
Consider using Reabsorption depending on circumstance. If they are good at concerting their attacks or they have some heavy combos (KTZ), this can bring a near dead ally to full health. Unless you play in a group with a healer, you will also often be the only source of healing at all on the team.
Level 16
Arcane Charge is a solid choice especially with the Spellpower boost, but Temporal Flux against a closer standing team enables you to shut them down repeatedly with Polybomb.
If the first does not destroy them, the second or third in the same fight might do.
Level 20
Lane clear is nice, but i would reserve it for PvE heavy maps (TotSQ) or when you are pushed to defense. Otherwise the heroic upgrades usually do better (especially Polybomb with fast jump-over.
That is straight not true. I´m playing Medivh often in QM and always pick him in ARAM when i get the chance and it usually works well. Reminding your party that they too can use portals at the start of the game works wonders here.