I guess in order to speak on this with real authority, one would need to have significant experience *as a proficient player* with all of the games that would be under contention for "toughest record."
Since I don't personally have that, I went with a more objective measure: the main reason that I cite Dean's score as the toughest is simply that it has the most "context" of any record due to the game having more competition than any other (by far).
Only on DK do you have dozens of players pushing the game as hard as they can, for years, all playing fairly regularly, and yet Dean's score is significantly higher than the 3 (very tightly-spaced scores) directly below it, which are themselves significantly ahead of much of the top 20.
If the competition on other titles were as intense and as incentivized as DK is, where people specialized at them over a long-term period, I have to wonder what kind of scores we would start to see on Mappy, Galaxian, et al. We have that kind of specializing in DK, yet there isn't even a contender for the next 1.2.