Poison (but currently playing USUM with a Salazzle)
Normal (they always end up being in the box)
Dark (i think? but will change when the time is right, i just didn't happen to use a dark pokemon ever SURPRISINGLY)
Bug (but currently playing with a Charjabug in USUM)
Weirdly, I was thinking about this very topic earlier today.
I think of all the types out there, the one I've definitely used the least is Ghost. It was only in Let's Go that I used Gengar, but I can't think of any other time I've used that type in my playthroughs.
Following that, my least used types would be Normal (as a primary type at least), Poison, Steel and Psychic.
I've used every single type, although some types like Bug and Fairy are less common on my teams than other types. When it comes to Bug-types specifically, I just find most of them to be weak unless they have a good secondary typing.
I have used every type but it wasn't until gen 7 that I used a normal or ghost type on one of my elite 4 teams, (Thought I did have a Smeargle on my HGSS team and a Bidoof on my Diamond team, they both got swapped out for other pokemon pretty early on.)
Probably the only type I have never used in a team is ghost, that's just a type I've never had an interest in training or mastering. Steel has mostly been limited to the magnemite family and empoleon, bug to Dustox up to the first two Hoenn gyms (and vespiquen for sweet scent in X), and rock to golem.
From Emerald until the last months of generation 4, so about five years my team structure was usually 4 Pokemon that were at least water or ice, a steel Pokemon and a Dragon/flying Pokemon.
Vaporeon, Crawdaunt, Kingdra, Suicune, Registeel and Rayquaza was the most common combination I used against my friends. We made up fictional (villain) teams that only used certain types
and most of us stuck to our teams for years. By the end of gen 4, the friend group had fallen apart and I really started getting into online and I introduced other types to my teams again. I still had my collection of water, ice, steel and dragon Pokemon until generation 5, but sadly I lost most of that when my first copy of Black broke. After that I diversified my collection of good/decent Pokemon for online use very extensively.
My memory might be fuzzy, but I don't think I've ever used a Poison-type Pokemon on my teams, at least not as a permanent team member. I used a Nidoking for a few days when I played Red version, but it didn't even know any Poison-type attacks, nor did I keep it on my team long enough for it to count.
That leaves Ice and Rock. I never took any Ice-types because there's almost none early-game except for really terrible ones like Smoochum, and none of the Rock-types like Geodude appealed to me, even with the wide variety at Ten Karat Hill. I also just left my Mystery Gift Rockruff in the PC for all of Ultra Moon.
I've also got my all-Galar team planned out for Shield, and I don't plan on using any of the new Ice and Rock-types like Rolycoly or G-Darumaka. Maybe on my second playthrough I'll pick up a Coalossal.