I've actually use a couple FTP clients, and I have to say Filezilla was the best. It was easy to manage all of the FTP servers and it was fairly fast. I don't think I would really need anything better than FileZilla.
I would have to say that FileZilla is my favourite. I can recall using SmartFTP back when it was freeware, and it certainly wasn't as easy or useful as FileZilla has been for me. For instance, with FileZilla it's easy to manage the files on the server, no messing around with some lame web-based frontend
I enjoy using the built in FTP client in Nautilus (on Ubuntu). It lets me mount my FTP folder as if a standard network share - so I can edit files directly on the server, saving time downloading + uploading.
Of course, I won't have a new backup, and if the server goes down when im writing to it..., and read/write times seem a little slow, but it is damn convenient!
I voted FileZilla, but I used to use FireFTP. It's an add-on for Firefox that opens up as a tab in the browser, but is just like any other FTP client. The layout is very similar to FileZilla, although there's some features of FireFTP that I like better than FileZilla.
I use D3lphin, (one of the two KDE file browsers). The main reason is that it integrates perfectly with the rest of my desktop, i.e. the ftp folder is simply another folder that I have open, making work easy. I have however used both gFtp and fireFTP (a firefox plugin) before.