I would guess Jigen is the most voted-on partially because he is emblematic of the whole group relationship. He has a clear dynamic with every other member of the gang (give or take Zenigata.)
Personally, I've always felt that it's Lupin that the other characters reflect off.
Jigen is sedate and humorless while Lupin is fun and outgoing.
Goemon is tradition and innocence while Lupin is modern and worldly.
Zenigata is responsibility while Lupin is freedom.
Fujiko was originally more about the male/female dynamic but recently the writers have played up the cutthroat, thrill-junky aspect of her personality so Lupin is now the restraint in that relationship.
But whatever it is, I'm impressed that these five characters meshed immediately and didn't need any additions to the cast. There aren't any Poochies or Cousin Olivers in Lupin's history.
Some of my favorite episodes are actually Goemon ones though, but it feels like he just doesn't play well with the rest of the cast, often getting sidelined in a big way when the story isn't specifically about him, while Lupin, Jigen, Zenigata and Fujiko are much better integrated into any kind of story.
I'm glad that the current writers not only seem to be thinking about who the characters are, but how they interact. Poor Goemon was sidelined way too often in the past since the writers either weren't certain what to do with him or didn't want him chopping his way to the end of the story in five seconds.
