5th edition is amazing. What school of magic did you specialize in? If you haven't picked that yet, my highest recommendation goes to Divination.
Which spell you pick is largely dependent on how your character is built. Flaming Sphere requires a bonus action to move, so if you're using bonus actions as part of your Bard abilities its utility is going to be significantly reduced. If you want a damage spell, I'd actually recommending picking Hellish Rebuke because it takes a reaction instead. Reactions are significantly less common, especially if you're staying at range and not frequently engaging in attacks of opportunity. Otherwise, I'd go with Shield if you don't already have it, for much the same reason. Shield will also block all damage from Magic Missile, and bumps your AC by a whopping five points until the start of your next turn against all attacks.