Speaking personally, I think we should specialize at least one or two of our Trismegistus slots, if not all three, into a single spell/potion apiece. I just get the sense that specialization will serve us better than generalization in our journey to scale up fast enough to stop You-Know-Who. Some that I think could be cool are:
Apparation: Just based on the blurb itself, I'm already pretty sold. Like seriously, all the different benefits sound so cool. Being able to cut through anti-teleportation wards is a very major benefit. If we can get to that level, a lot of options open up. Like, just as an example, we could cut through Malfoy's (or other Death Eater's) wards and torch their place with Fiendfyre. The increased teleport distance is also very nice for coordinating with our Hufle-network throughout the country. Plus, if we get good enough at it we can set up a Moonbase that we can teleport to/from that we can be fairly certain The Dark Lord won't be able touch. It's just cool, okay?
Fiendfyre: The ultimate attack magic, pretty much. Or at least, the best not counting Avada Kedavra. But Fiendfyre can do AoE and probably non-lethal (just tell the fire to partly burn people instead of totally immolate them), so I count it as better. We very much benefit from an increase to our ability to kill stuff dead, and the fact that this is one of the few things that can affect Horcruxes makes it quite valuable. Additionally, there may be some noncombat applications with doing like potions or alchemy or smithing with a Fiendfyre flame, but that's just some guesswork on my part. Also, the fire has its own will, so there might be some benefit there if we're really good at casting it? I don't know what such a thing would look like, though. Might be some issues finding a way to learn it and/or cast it without getting thrown in detention/jail, given that it's pretty Dark. Unsure how much of an issue that'd be.
Patronus: Honestly a pretty good pick, I think. It's already a fairly versatile spell in canon, being able to defend against some Dark beings, serve as proof of identity, and pass along messages for its caster, so I think it'd be interesting to see what a boosted one is like. Has synergy with our House Boon, as has been discussed, and a boosted Patronus may have some effectiveness against He-Who-Must-Not-Be-Named like I pointed out above. Also, I imagine a boosted Patronus would be like a Stand, and that's all kinds of fun.
Protego: We're immune to the stuff that just goes through a Shield (the Unforgiveables), so having a super-strong shield that can block everything else seems conducive to our goal of not dying. Not much to say, other than the dramatically increased survivability.
Finite Incantatem: Lmao, get Finite'd. A super-boosted general-use counterspell seems like it'd be the kind of thing to come in very handy when our life is going to be under threat from all sorts of magical things/beings. Again, not much to say other than that being able to just go "NO" to any magical effects we dislike would presumably be incredibly useful.
Felix Felicis: Man, it sure would be lucky if this next training/study session had some incredible and relevant breakthroughs to the subjects at hand, wouldn't it? We'd have to use it sparingly to avoid side effects, at least at first, but we should be able to get the ingredients through our Huffle-network and it'd likely have an extremely potent effect. Hard to speculate on what becoming amazingly good at temporarily giving yourself unbelievably good luck looks like.
Not an exhaustive list by any means, but I think any of these could turn out fairly nice if chosen.
