all of the games are good options!
pikmin 1 is super short compared to the others, but pikmin 2 is BY FAR the longest, and is my personal favorite -- i'd say it probably has twice as much content as pikmin 3, but it also can be the most difficult. mechanically, pikmin 3 probably has some of the most fun gameplay (i love getting to multitask by managing 3 captains at once), but the game felt like it went by too quickly, at least for me
personally i think starting with the originals is the best way to be introduced to the series (and you can better appreciate all the new kinds of pikmin added in the different games, rather than feeling like something's missing if you play them in reverse order)
(another thought: pikmin 4 kinda looks like it's bringing back a lot of mechanics from pikmin 2 (like caves, treasure, upgrades, so pikmin 2 might actually be the most "similar")