Alabasta is influential because so many strong characters were introduced. Also it was the first time Luffy faced an opponent who truly outclassed him in the form of Crocodile.
Skypiea however is the most influential in my opinion for several reasons. So I find myself agreeing with those who have said this before.
First, the experience is unique, unprecedented and most importantly something exclusive to the Mugiwara crew. Its not something the WG knows about, nothing that added to their notoriety, just a pure unadulterated ADVENTURE, in short the very essence of why everyone became pirates in the first place.
Secondly, it gave everyone a massive chance to power up and develop as fighters, especially the non-elite fighters like Nami and Ussop who have benefitted immensely from the Skypiean tech like the dials.
Third, it is the first time Luffy and Co single handedly saved an entire civilization from extinction. Arabasta was well and good but the unique nature of sky island and its people makes it that much more special. And the threat they faced was that much more monumental.
Fourth, it is one of the few things that the Mugiwara crew now share exclusively with Roger. The connection was always there but the Skypiea adventure truly cemented the links. Robin's discovery of the Poneglyph and the writings of Roger, the Will of D etc all were destined. It was that single incident which has inexplicably thrust the Mugiwara crew into the path of Destiny.
Fifth, the gold…oh the gold. Its the first and so far only arc in which the Mugiwara's actually acted as Pirates and took loot (even though it was given willingly) This simply cemented the story as the pure adventure it was.
Sixth, it indirectly led to the whole Water Seven fiasco. Think about it, if Skypiea hadn't happened the Merry would not have been damaged to that extent. The crew would then have had no reason to visit the Galley La company, meet Franky and purchase Sunny etc. Merry's loss was sad yes, but it was a necessary step, and given the revelations about the Oro Jackson and such also furthered the legend of Roger and the connection with the Mugiwara's.