Although I would like to claim War Caster would be the best pairing, War Caster is only useful for maintaining concentration on spells and for using spells as a reaction attack. In truth, the best Feat you could pair with Spell Sniper would be Magic Initiate. With Magic Initiate, you gain two spell cantrips from a magic class and one of the first-level spells from that class. Should you play your cards right, you can end up doing an absurd amount of long-range damage using only cantrips.