I don't think any letter is really useless; I like all of them, I guess. But "q" can be eccentric, since it either necessitates a "u" right after it or just acts like a "k" in disguise. However, I like how the letter "Qq" looks as opposed to seeing "kw" like that every time.
"X" is an awesome letter, but frequently it has to be preceded by an "E" or just mimics "Z" in disguise.
When letters act like other letters in disguise, it gets on my nerves because you're like "What the heck is the point of the other letter existing, then?" I guess for a change in looks, but...
Finally, silent letters, like in the words "euphemism" :cer_pissed: and "gnat". "Uphemism" and "nat" would be perfectly fine (though the former might be mistaken in pronounciation for "uh" instead of "yoo"). The "G" in gnat just seems like the word inventor just stuck in a "G" to screw everyone over to no end...
I can think of plenty of useless words, though. I dislike it when you can simply swap out synonyms, and people marvel at the "diction" in an essay because you switched out some words. All you really did was use a lesser-used word. Even though I employ this technique to improve my writing language, the technique still doesn't sit well with me. But let's not veer off-topic...
So, for me, silent letters, letters in disguise, and letters that constantly require the use of other letters are perceived as not very useful...