Mine are, in no particular order:
1 - Augury. Fortune telling is, naturally enough, difficult to pull off in an RPG. Augury in its AD&D incarnation is a surprisingly elegant way of doing it.
2 - Hold Person. As a DM I love this one for hostile magic-users - out of pure sadism. I think being immobilized and not able to do anything is almost the most annoying thing that can happen for a player in a game (even more annoying than death in some cases). And sometimes it is fun to annoy people.
3 - Hallucinatory Forest. I have never seen it used, but there is something endearing about this spell. Every now and then, amidst the melange of different influences making up D&D, you catch a glimpse of something that looks like it came out of a fairy tale. Hallucinatory Forest is one of those moments.
4 - Nystul's Magic Aura. Almost pointless, and yet...and yet...you can imagine situations in which it could be used, and such a situation would be really cool. Like using the magical aura to leave a clue for an associate in a cloak-and-dagger scenario, or to mislead an evil wizard, or something. It's the kind of thing you look at as a pretentious 12-year old DM and your mind explodes with all sorts of interesting ideas that you know your gaming group with its 12-year old members will never get anywhere near doing, because it's nothing to do with killing orcs.
5 - Prismatic Spray. For sheer FUCK YOU value, there's probably not much better than this spell.
6 - Leomund's Tiny Hut. If only Scott had had this available to him on his journey back from the South Pole. I love the idea of a bespoke spell purely to allow PCs to survive in hostile environments, simply because it implies that that's the kind of thing PCs might want to get up to.
7 - Magic Mouth. Like Nystul's Magic Aura, Magic Mouth is almost worth its own existence just for the uses you can imagine for it. Also, who can resist the idea of a rock or table or vase suddenly sprouting a mouth to spout a message to somebody? It is pure, childlike fantasy, and there is nothing wrong with that.
8 - Cacodemon. Demon-summoning is one of my favourite fantasy tropes, so this has to close off the list. As with Hallucinatory Forest, the existence of this spell hints at something altogether different from the typical genre influences of D&D, except this time it's Dr Faustus rather than Brothers Grimm.