Ideally no one, and if the cost / expertise is so niche that only a handful of sophisticated actors could possibly actually do it, then in fact (by way of enforceable treaty) no one.
> enforceable treaty
How would you enforce it after you get nuked?
We're talking about making war slightly more expensive for yourself to preserve the things that matter, which is a trade-off that we make all the time. Even in war you don't have to race for the bottom for every marginal fraction-of-a-percent edge. We've managed to e.g. ban antipersonnel landmines, this is an extremely similar case.
> How would you enforce it after you get nuked?
And yet we've somehow managed to avoid getting into nuclear wars.