For Artillerist, I'm fond of Dragons Breath with a non-fire element to complement the cannon. Its not as much boom as fireball, but it can be used with less risk to allies. It can be used on familiars (Magic action <> attack)
Since it fits in the SSI, it can be used pretty widely. Nothing like your two front-liners and a bat-familiar starting a combat by breathing an array of elements. If you've got an archer or other non-caster, let them maintain concentration.
The GM was very cross when we rescued a wagon load of slaves-to-be and my artificer had them take turns using the SSI to cast DB on several PCs before hiding behind the wagon.
Though I agree Aid is great one to spam with SSI, especially with familiars, steeds, and constructs around. Its "only" 5hp, but since it raises hp max, those can be recovered during short rests, by potions, healing checks, etc.