I haven't read The Hobbit, so I can't make a full opinion on Bilbo. I do think he's catty and funny though, and just overall a great character in Fellowship.

Definitely agreed about Sam, though. He's my favorite Fellowship member after Merry (who is so much cooler in the book than in the movies).

I would have to say, though, that it's really hard to qualify who exactly was the most important member of the Fellowship. Of course secondary members like Merry, Pippin, Gimli, and Legolas probably aren't going to claim that title. That being said, they all played important parts in the war (Merry and Pippin especially) and I wouldn't call them unimportant. Even Boromir's involvement was important. I think the individual roles in this series are so closely intertwined with the events in the later books especially, that you can't really say that any of the characters could have been easily removed without changing huge chunks of series, and the ultimate outcome of the series itself.