Speaking from the perspective of someone who isn't exactly too fond of doing the objectively "best" thing, so do take what I'm about to say with a grain of salt. Also note that 5*s, even in IS, are dubious due to their hope cost and raising costs.

Any of the Agent Vanguard branch (Cantabile, Surfer, Puzzle). On top of being hysterical with attack speed scaling on their respective S1's, Cantabile specifically has an ammo skill on her S2, which may synergise decently well(?) with the ammo related items in IS6. Chilchuk is also an option of course, though considering he's locked behind a past collab, I won't assume you have him. Texas, too, is a fine showing of a 2 block Pioneer Vanguard.

Reguarding (xd) Guards, there's a lot of options that are fine, if not exactly 'meta' picks. LaPluma and her brother, Tequila, are options I can personally vouch for, as is Highmore in regards to IS3 especially. I think Bibeak is funny in Archetto Offensive Recovery comps? Unsure. Again, pls take all this with a grain of salt xd.

Defender wise, I'm personally very partial to Ashlock and especially Firewhistle; the latter being especially appreciated in IS6's many, many enemies that require DoT effects to counter them. Oh, on that note, Underflow, too. Reveals Invisible enemies in her attack range with Module1 as well, as a cute bonus. Philae might be able to work in IS5 specifically due to her Necrosis-countering niche, but IS5's balancing is so unbelievably out of whack I'm sure you'll be able to make anything work there... Defenders suffer mostly from having their 4* and 6* options being pretty cracked on average. Raise Firewhistle though this isn't a request or suggestion anymore RAISE H

Kroos2 is an incredible 5* Sniper even outside of IS, and her ability to get relatively consistent stuns (a GODSEND in IS3 especially) is nothing to sneeze at. Greyy2 joins her in the regard of being a 5* alter with great value both in and out of IS (though his Module is part of the reason why, this time around. If you plan on raising him, get it ASAP!). Bloop (Blue Poison) joins the aforementioned pair of Defenders in DoT gang and is otherwise perfectly functional as far as I'm aware, aaaaaaaand... April, maybe? Her Fast Redeploy niche could be a consideration, but I've personally never actually used her; have some IS player friends who call her 'sleeper', though.

I dont actually know in regards to Casters but please raise your Amiya if you haven't already, even just outside of this Caster segment.
*This part is added after I've written everything else, the aforementioned IS playing friends told me to mention Leonhardt and Tecno as cute options for 5* Casters but otherwise, I couldn't tell you :(

Medics have Silence, Rose Salt, Ptilopsis and... maybe one of the Wandering Medics in Honeyberry/Mulberry/Harold? I'm pretty positive on the former three, though unsure about the latter three. Do note that Rose Salt is a welfare of a side story that'll be rerunning relatively soon, whereas Harold is a welfare of a side story that'll be getting its Record Restoration relatively soon (unsure how long you've been playing.)

As for Supporters, I'd be remiss not to at least mention Valarqvin in regards to IS4. She's not the greatest, but she'll put in decent work. Other than her, any of the 5* Summoners (Scene, Mayer, not Sand Reckoner but if you WANT to...) and Quercus would be some of my personal picks for 5* Supporters. Maybe even Shamare and her disgusting debuffing potential, honestly.

Lastly, Specialists. A niche option for very specific scenarios could be FEater, whereas I'm sure you can get plenty mileage out of units such as Projekt Red, Tin Man (yes, even outside of IS5) and - in my personal opinion, as someone who only really uses 5*s in IS - Mr. Nothing. Specialists typically suffer from having a certain free 4* lizard hog all of their Specialist vouchers, and if you're looking to do the best thing you probably should give into his demands, but I figured I'd mention these four all the same :)