There are a lot of good spells out there, but as for "underrated" spells which conventional wisdom says are bad but are actually good...

Longstrider is the only one that springs immediately to mind. A lot of guides seem to view it as mediocre, but in my experience it is quite good in many situations if your party knows how to use it. Here's the key observation: opportunity attacks are usually worse than full attacks. (Because neither multiattack nor Extra Attack is usable on an opportunity attack, and incidentally neither are Push and Grapple.) Longstrider is a 1-hour no-concentration spell, castable on allies, which scales with spell slot level, that makes you fast enough to convert attacks from most monsters into opportunity attacks. (E.g. attack in melee, then move 40' away. The monster can move 30' toward you, or move 60' on a Dash, but it can't make a regular attack against you this turn. The opportunity attack on your turn is the only attack it gets.) That can cut damage by 50%, which is sort of like turning into a barbearian.

(It's not a spell, but the (Hunter) ranger's Escape the Horde ability is good in a similar way and combos well with Longstrider.)

Hmmm, what else? [thinks]

Otto's Irresistable Dance is underrated by some sources, including TreantMonk's 5E guide, which rates it as "terrible." IMO it's very, very good for any party with a ranged capability, especially if you rely on summons. It bypasses legendary resistance and forces the enemy to either lose an action getting free, or stand immobile. Against e.g. a (vanilla) ancient dragon with a breath weapon range of only 90', being immobile is the same as being rendered ineffective. Its primary deficit is its short range, but see my next paragraph...

Phantom Steed doesn't show up much in chatter, but a steed with 100' movement means that you can get 200' of free movement on your turn under 5E's mounted combat rules (by making your steed dash), or 100' with a free disengage. What that means is that you can, for example, start 130' away from an ancient red dragon, make your Phantom Steed mount move to 30', cast Otto's Irresistable Dance to nail that dragon to the floor, and then have your mount Dash away back to 130' so you're out of breath weapon distance. Meanwhile, everybody else has free advantage to attack the dragon with their melee or especially ranged attacks--which can cancel out a range penalty if your skeletal archers are dispersed over a wide area for protection.

(And that, my friends, is why I don't run vanilla dragons. All my dragons have Dragon Sorcerer levels, so killing them becomes a lot more complicated than just hitting the dragon with Otto's and a bunch of skeletal archers and giant owls. The dragon could Counterspell your Otto's, or do a Quickened Dispel Magic, or a Quickened Dimension Door... the uncertainty of facing a spellcasting dragon is way more fun and interesting than just fighting a bag of HP like the vanilla dragon.)

-Max