Boon of Magic School Mastery = Divine Smite every turn for free. But what L7 spell to take?

About to hit lvl 19 Devotion Pally in Eve of Ruin, and I'm not seeing a better Epic Boon feat than Magic School Mastery. +1 to CHA, choose evocation, pick Divine Smite. Now I get to smite every turn w/o using a spell slot, which is also constant half cover for everyone in a 30-foot radius. Seems like a no-brainer to me, but I'm not quite sure what spell to take for the Signature Arcanum. Unlike for Rote Casting, the feat doesn't specify that the Arcanum needs to come from the Paladin spell list. So...Delayed blast fireball? Whirlwind? Or something lower level? Wall of Force? Holy Weapon? Mass Cure Wounds? 

It does have to be evocation, but yeah any level. Crown of stars is good if you're not flush with BA options. Prismatic spray is great to soften em up on round 1. many solid options.

What is going to complement what your char does now?

I’m overflush with BA options, unfortunately, since smite still requires one in 5.5; the feat just makes it not require a spell slot. 

Keep in mind that the spell slot-less version of smite will always be at level 1. That’s the way all free castings work, you get them at the lowest level possible. So, yes, you can do it, but at level 19, a level 1 smite is pretty small potatoes.