The worst for me is Vaan because I found him to be annoying and whiny like Tidus. He never really stood out as a great main character to me and he seem pretty weak to begin with. Penelo comes in second because her role in the game was pretty useless. She remind me of Rikku too much who I also dislike. You would think when they made this game that they wouldn't create the characters to look like a recycle cast of Final Fantasy X.
I can't see too much of a similarity between Tidus/Rikku and Vaan/Penelo to be honest.
Ok, Vaan and Tidus both have their foot in mouth moments, but Vaan isn't actually that whiney to be fair. On the whole I prefer Tidus because his whole story and background where alot more indepth than Vaan's. But anyways, they aren't really that alike.
With Rikku and Penelo aswell, Penelo is a HELL of alot more mature than Rikku will ever be, she keeps a close eye on Vaan to keep him out of trouble and she cares deeply for him. With Rikku, she shares more personality with Tidus then any other character, at times she does come across as a female version of Tidus. This is not the case with Penelo and Vaan.