I was wondering, what is the rarest light bulb in history?

I don't really think that question can be answered. It could be any lamp that there is only one example of in the world, or it could be the one that took a collector the longest time to find; there are any number of ways to describe the 'rarest' something.

Whichever way you choose, how would you go about finding out..?

I guess a /X MV is sorta rare, besides that I’m sure there’s something rarer but I don’t know.

I’d class rarest as something like GECs ‘golden’ linear lamp?, something that wasn’t in production long due to its high cost to make maybe?

Figuring out the absolute rarest bulb might be a bit hard but amongst the designs that made it into a production, these might be one of the rarest made:

- The F65T17 lamp, this was a 3ft long 65w T17 lamp that had a pretty shortlived production run from around 1941 to 1945. There are a couple of examples posted on LG.

- The shortlived /X mercury lamps. These were a colour corrected (/C) with a special purple stain in order to improve the colour rendering qualities. Replaced by the /DX mercury lamp.

- The Caution Yellow /Y mercury lamps, these were yellow stained colour corrected mercury lamps that gave a yellow glow and was used to highlight important areas on roadways. The introduction of HPS rendered these more or less not needed.

-The 450w H40 mercury lamp. This was a mercury lamp designed for use on 480v circuits with a reactor ballast so it had a higher arc voltage. The 400w H1/H33 mercury lamp proved to be the more popular lamp in the 400-450w range.

The 10W and 200W SOX lamps seem rare, but I'm sure there are more lamps that are more rare than that.

10W SOX are rare, but they do come up. I've never seen any of the pre- re-rated ones though. The rarest SOX has to be this little one, which never made it into production

  I  have a very rare westinghouse british made GLS carbon filament 40’watt GLS