It's the only fictional profanity I've EVER come across that actually works. Everything else seemed obviously and cheesily "made-up". Perhaps it's just in the performance/delivery.
I'm going with 'frack', as it sounds more harsh than the others (even if ruttin' is real slang.) As much as I love Farscape, 'frell' and 'dren' have alwasy been sorespots for me as they're too pleasant sounding, even when spoken harshly. They're too pleasant sounding to be used as gutterally as they are.
Also, 'frell' sounds like a shampoo.
Personally, as a write in, I'd say that the Mandarin curse words used on Firefly are the best, as they're all real Mandarin curse words.
I based one of the main villains in my homebrew on Bloth (made him a Blackguard Liche), and tried to figure out how to make a Ecomancer PrC based on Tula
Another one is "Lucy" from the novelizaton of Douglas Adams's Starship Titanic by Terry Jones. I'm not familiar with the game, so I don't know if it's there too.
I tend to think that the idea of a bad word is quite foolish. If a word's so bad don't put it in the language to begin with.