I never played the first one until it was on the Expansion Pack, and I feel it doesn't hold up well. 2 recycles most of the minigames anyway, the harsh nature of the luck factor is too much in 1 (2 and most later games balance this better). 10 is the only main series Mario Party and perhaps the only Mario Party I find worse than the original.
I rented 2 for the N64 as a kid and I played it on Wii VC. 2 is one of my favorite Mario Parties.
I haven't played all the boards yet or every last minigame in 3 yet but I've played most boards and such and it's the best on N64. I only played one round of it as a kid on N64 but have played hours of it on the Expansion Pack.
As for the series overall, 6 is my favorite. 5 was for a while (I like the side modes) but Party Mode and minigames in general are better in 6. Superstars is one of the best in the series as well.
Lifetime Sales Predictions
Switch: 161 million (was 73 million, then 96 million, then 113 million, then 125 million, then 144 million, then 151 million, then 156 million)
PS5: 115 million (was 105 million) Xbox Series S/X: 48 million (was 60 million, then 67 million, then 57 million)
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima