Definitely aura. The medstation is the best healing ability by far. Plus she’s really fast, but doesn’t suffer from the same very good aim requirement as sparks to be decent in combat.
Sparks is my second place, reviving someone across the map before they hit the ground is awesome. Plus instant healing medpacks and more fast movement. The only issue with her is her difficulty in combat.
Guardian is my third. She’s not really my meta pic for medic, but she’s really fun when played as an assault who happens to have a revive ability. Plus, annoying arty mains and freerotation naders. Her revive can be kind of annoying sometimes though. That plus the cooldown keeps her below sparks for me
Phoenix is cool too, he’s not bad, but it does feel like he’s on perpetual cooldown, unlike other medics who can either pick up their medstations or have multiple medpacks. He’s still fun, but his healing cooldown bumps him down for me
Sawbonez is my least favourite medic though. He’s just so boring. He’s not bad or anything, but only throwing medpacks that take forever to heal without potent packs is eh. I’d rather play another medic or another assault, not this bland hybrid. At least guardian is interesting for an assault hybrid