C/3

Conjure Barrage

Do you like to throw a dagger at people?* Do you like to throw, like, two hundred daggers at people? Well, then, this is the spell for you. It's another low-damage, high-save-potential spell, and it isn't going to do that much in a real-world setting but look cool, but, I mean, it's going to look really cool. 

Real-World Rating 5 (Effective But Limited)

Conjure Celestial

Since we're in the "Conjure" section of the alphabetical listing of spells in the Player's Handbook, be prepared for a bunch of "see above", but here's the basic situation with summoning spells. Although they're hit-or-miss in the D&D game itself, they're completely devastating in the real world! Nobody has any experience with celestials or any other kind of non-normal creatures, and a lot of them require magic to even damage them. And guess who has no magic in the real world? Everyone! Everyone but you!  Even at low levels, this is creating a gigantic headache for your opponents; at high levels, as with this particular spell, you're basically pulling a literal angel out of thin air to fight for you. And do you know what angels are famous for in the Bible?  That's right: killing a shitload of motherfuckers.

Real-World Rating:  8 (Very Effective)

Conjure Elemental

Our first "see above". In D&D, what is an elemental? Depending on its level and the level of your PCs, it's a minor annoyance at worst, and a tough but fair enemy at best. In the real world?  It's a giant monster made of fire and you can't do anything to stop it. 

Real-World Rating: 9 (An All-Time Great)

Conjure Fey

Fey don't get a lot of play in a lot of fantasy RPGs, probably because of the name. But if you read a lot about Celtic myth, they were absolutely terrifying. I don't really have an opinion about this spell that diverges significantly from my opinion about other conjuring spells, but this is perfect if you want to create, say, an army of horrifying magical kidnappers. 

Real-World Rating: 8 (Very Effective)

Conjure Minor Elementals

The primary difference between Elementals and Minor Elementals is that the former use the latter as talent feeders, and small towns are more likely to have Minor Elementals that are more affordable and family-friendly.

Real-World Rating: 7 (Effective)

Conjure Volley

This is the ranged version of Conjure Barrage. Depending on how we settle the question of whether or not guns count as standard missile weapons under the 5E rules, this means you can either do a bunch of arrows (okay but not anything you can't do by hiring a bunch of out-of-work Olympic archers) or a full-on Bullet Hell scenario. 

Real-World Rating: 6 (Pretty Okay)

Conjure Woodland Beings

The conjuration spells go out with a whimper rather than a bang. This is still fey creatures, but the silly name limits how impressed you can get with this spell, and it can only be cast in an area where these beings exist naturally. Believe me, folks, if you're out by yourself in the middle of the forest and need to murder someone, just using a rifle is much simpler and more effective.

Real-World Rating: 4 (More Trouble Than It's Worth)

Contact Other Plane

Another great example of a classic D&D spell that sounds a lot better than it is. This basically lets you make a direct call to a god, a ghost, or some other kind of extra-dimensional whatnot, and ask them a question. The answer can only be one word -- a mixed blessing, as the entity can't give you any complex or multifaceted information, but also can't ask you why you're wasting its time, effort, and massive divine power on your rinky-dink little scheme -- so there's not a huge upside. The downside is that you might go incurably insane. You're better off just Googling.

Real-World Rating: 2 (Mostly Pointless)

Contagion

Sure, you could waste a 5th-level necromancy spell on giving your enemies a horrible disease. Or, you could just hang around with a couple of flunkies who don't want to get a vaccine because "freedom".

Real-World Rating: 4 (More Trouble Than It's Worth)

Contingency

Contingency is one of those spells that rewards you for being a weirdly calculating nerd. Traditionally, it gets ignored, because it's a high-level (6th) spell that doesn't have an immediate or spectacular effect; all it does is make something happen if something else happens. That's not sexy! Who would fuck around with some if/then nonsense when you could do Eyebite or Otto's Irresistible Dance? But there's also a small group of insidious weirdos who figure out some way of having Contingency result in a seriously insane chain of actions that can make for some of the most spectacular RPG blow-ups you've ever seen. This would be a great spell to take in a world largely devoid of magic, because you could build in an entire series of other spell effects triggered by very banal, predictable reactions, so don't write it off just because of its boring concept.

Real-World Rating: 8 (Very Effective)

Continual Flame

A second-level spell that costs 50 GP to activate and is the functional equivalent of owning a cigarette lighter. Hard pass.

Real-World Rating: 2 (Mostly Pointless)

*: Trick question, nobody likes doing this.