1 seemed like a slightly rushed launch gamecube title (well it was)
2 was a an improved version of 1 with greater depth and more focus on exploration.
3 was trying to improve on 2 but seemed much shorter again (to me at least)

so I'd have to say 2 was my favourite.

Pikmin 2 was the best out of the original GameCube Duo. I haven't played nor bought Pikmin 3, so I can't really comment. However, it has a lot to live up to, I'm counting on it to be really good.

Pikmin 3 is my fave..
HD graphics- it looks stunning
Gamepad- the map is very handy for keeping track of things, especially when multitasking. Off tv play too
DLC- yes the dreaded DLC, but to be fair Pikmin 3 DLC offers great value for money. It has replay value galore and really helps prolong an already great game.

the last obviously!

I'm all about the 2. So many people hated the caves, but I freaking loved them. Pikmin 2 wins in sheer content, and while some might argue that the content was repetitive, as long as I'm playing Pikmin for hours and hours I'm happy. Amount of content is important to me, and Pikmin 3 was great in ways but pretty disappointing in this regard.

Pikmin 3, THE BEST?

I can't understand why, this game it's more like a reboot of the original Pikmin, it reminds a lot of it.

Pikmin 1 it's good, However, since it's the first of the series, naturally it has the worst gameplay mechanics and worst concept. Collecting ship parts in just 30 days it's just terrible, makes the game depressing.

Now, for me, Pikmin 2 it's clearly the best, since it's the longer, it's the game with more content, awesome, yet simple story and great Zelda-like dungeons. It also has the Piklopedia, and this is possible since this game has more enemies, something that Pikmin 3 lacks. But it had the fruit, I can't understand why did Nintendo decide to make those bland descriptions of the fruit on the Gamepad, it's disappointing when you compare it to the Treasure Hoard. This thing had very clear and awesome descriptions of the treasures from Olimar and the ship.

SPOILERS:
Pikmin 2 also has the waterwraith, wich is way cooler than the plasm-wraith in my opinion, despite from being weaker. That sense of horror and suspense in the Submerged Castle is awesome, best dungeon I have played on my life.

Pikmin 3 is the best one in my opinion. It has the best story mode and the multiplayer is also tons of fun! Hopefully it reaches 1 million! =)

Pikmin 2 is the best, followed by 3