Gen 1 Kanto: 45.69m (baseline); Pokemania unmatched.

Gen 2 Johto: 30.12m (-34%); The steepest dropoff.

Gen 3 Hoenn: 23.28m (-22.7%); Continued dropoff.

Gen 4 Sinnoh: 25.27m (+8.5%); Relative stagnancy.

Gen 5 Unova: 24.16m (-4.3%); More relative stagnancy.

Gen 6 Kalos: 16.76m (-30.6%); Another very steep dropoff.

Gen 7 Alola: 25.56m (+52.5%); Revival back to gen 3-5 levels.

Gen 8 Galar: 26.04m (+1.9%); Relative stagnancy again.

Gen 9 Paldea: 25.69m (-1.3%); More relative stagnancy again.


Observations:

Steepest drop off: Gen 1 -> Gen 2

Lowest point of the series: Kalos

Biggest comeback: Alola


We are ignoring remakes here because only the first 4 gens have remakes so far. But if you want to include remakes too, here's the situation so far:

Kanto (RGBY & FRLG): 57.69m

Johto (GSC & HGSS): 42.84m

Hoenn (RSE & ORAS): 37.91m

Sinnoh (DPPt & BDSP): 40.33m

Unova (BW/2 & ???): 24.16m [remake pending]

Kalos (XY & ???): 16.76m [remake pending]

Alola (U/SM & ???): 25.56m [remake pending]

Galar (SS & ???): 26.04m [remake pending]

Paldea (SV & ???): 25.69m [remake pending]

If we included LGPE for Kanto too it would be even further ahead, but I think LGPE doesn't count toward the normal pattern of remakes. Will there be future "let's go" games? But if you want to include it, then Kanto would be at 72.76m.


It's worth noting, however, that the video games industry as a whole is about 10x bigger now than it was 10 years ago, and about 100x bigger now than it was 30 years ago. When you adjust for this, you see how special late-90s Pokemania really was.


Of course now the most popular pokemon game of all time is Pokemon GO, and by such an enormous margin that it is actually more popular than every other pokemon game in history combined.