Fallout 1/2 are the best rpg's of all time regardless of how many walls of text people write.
Only arcanum is on their level
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But are they the most reactive?
Probably not but it depends how you define reactivity. Which is hard to fit into a mold.
I see people posting AOD, which has a lot of choices and consequences but the game world doesn't react to you in an organic sense. You do things, and you go down paths, and maybe that's reactivity or maybe it's not.
jackofshadows , if nuking a city is reactivity then fallout 3 is also reactivity.
In fallout 2 if I join the bishop crime family, the mordinos/salvatores/wrights are how hostile. Is that real reactivity? although it's still somewhat limited and more along the lines of 'faction reactivity' like you see in new vegas or games like that.
Same thing in underrail choosing the black eels or later the jkk or protectorate ect.
In baldursgate 1 if you help the druids in cloakwood later on some guys brother tries to kill you, or you can help him do some quest if you keep him alive.
When I played might and magic 8 and joined the dragons and wiped out the knights at garrot gorge is that reactivity? If so then might and magic 8 has about as much reactivity as any of the games above.
To me the gold standard of reactivity is fable. You can choose your name, I liked chicken chaser. People in the game world react if you fart next to them, no other game offers that type of reactivity.
Those are real reactions, not playing make belief like some people in this thread