>>54714493judging by defensive teras on the strategydex, the best types defensively would theoretically be
1: Fairy by a wide margin because of how rare physical offensive poison type moves are as coverage, it gives resistance to dark coverage like knock off which is everywhere in gen 9, and neutral to common attacking types like ground fire psychic etc.
2: poison is easily the second best despite being weak to two really common attacking types simply because they can absorb Toxic spikes and they cant be hit by toxic. It resists fairy and fighting which is massive for a large amount of OU pokemon that are weak to it. it shares the same thing with fairy where one of its weaknesses (psychic) has basically 0 physical representation except a stray zen headbutt coverage slot on something. this makes spdef tanks with poison tera much harder to take down. iron valiant and clef both get shut down by this tera too
3: Steel is probably the most common tera type after water on both offensive and defensive pokemon this gen. Resists stealth rock and its toxic immune and it reverses a lot of type match ups for the original pokemon.
4: water, it just has a lot of favorable neutral match ups, thats it.
It seems like fairy is a priority for avoiding dark type damage and having scarce weaknesses, im suprised that poison tera shows up so often in defensive pokemon analysis pages on smogon and even my own team builder considering psychic and ground are rampant in OU but i suppose that at least 1 grounded poison type is required on each team to not insta lose to toxic spikes. Steel being that high is no surprise considering fairy steel was the best typing and teams had no problem running magearna and mawile on the same team for years as a core. i would say steel and fairy is probably the best defensive type while ghost fighting or ground flying are probably the best STAB typings for offensive pokemon
