Most important mapslot?

Human memory compresses experiences according to the "peak/end" rule, which suggests that the finale is the most important as a distinct slot.

In practicality you need the first few maps as a "hook" to keep the player entertained enough to make it that far, though.

You can probably target your "peak" experience for any map slot, but I find it feels most natural around the 2/3 to 3/4 mark.

I haven't made a 32 level megawad in a while.
So speaking of the latest one of these, Plutonia 3: I had put the most freshly made for warming up maps as 1-3 (btw the maps 4 and 5 used to be 1 and 2 ones respectively)
The best Zdoom map demakes were put between 21 and 28. I think map23 should be the best, and map27 - the second best in the whole wad. Map22 used to be Map01 of zdoom pl4, I had a lot of motivation to draw it just cuz it should've been a fresh start then. Map21 also was an episode starter in zdoom pl3.

In Boggy Region (24 maps + 2 secret ones) I think the best was map16.

My most recent almost gonna-be-a-megawad thingy has the best one in map20 slot.

If I had fresh 32 maps ready, for some reason I think I'd put the best map in slot 8, 16 or 23(maybe 24), I dunno why. If I had more than 32, it'd be 35th.

Согласен
На мап02 хорошо быть, когда твоя карта в мегаваде, где остальные карты не твои. Очень высока вероятность, что в неё поиграют

MAP01, MAP07, MAP10, MAP11, MAP21. Also MAP29 and 30.

The most important map slot is MAP01. It doesn't have to be the best map or the prettiest but it should probably be some of your most competent, plotted out work. It ought to lay the foundation for the wad's themes immediately and clearly, and sometimes as simply as possible. It should probably be on the shorter side of things but dense enough to pack a punch that will propel the player into playing through at least the next few maps.
Keep it streamlined, don't get too experimental, you can get away with that in later maps. First you have to win the player over with a lean proficient opener so they trust you and won't immediately discredit an unorthodox map as unfair or unclear. You need to show them that you know what you're doing and won't pull any clumsy bullshit on them. First impressions are everything and a MAP01 that feels even a little amateurish can be a sign to a player that the wad isn't worth playing. All these years later E1M1 stands out in our minds and for good reason.

For a full megawad, I think that maps 1 and 2 are the most important. If these 2 maps suck, then the rest must be garbage too.