Panic's "Transmit". Hands down. Only downside is that it sometimes truncates long filenames of downloaded files, but resuming is not broken, even if it does so. Second place goes to the CLI ftp client ncftp.
FREEING ROBSTAHS!!!@@ RUN ROBSTAHS, GO, GO, ROBSTAHS, RUN FOR FREEDOM##@! DOMO-KUN SAVE ROBSTAHS;;