Which lens would you recommend?

[OzOutbackGirl]

HI everyone, I can't decide which lens. Have been procrastinating for weeks now. Maybe you guys can help me. I currently have a Canon 350D (will upgrade to the 5D in about 6 mths though) and 2 kit lenses (18 - 55 & 75 - 300mm) and a Sigma 18 - 250mm. I am not sure if it's the lenses or me but my photos all seem a bit soft and not really tack sharp. The Sigma is ok up to about 200mm. Then my kit lens 75 - 300 seems better 200mm and more when I take animals but still not sharp. Is this normal?

Anyway, I really want a lens that is a really good for landscapes, sunsets etc. I was seriously considering the 24 - 105mm but have read such good reports on here about the 15 - 85mm or the nifty fifty? Which one would you suggest I purchase as a good everyday lens I could take hiking and basically photograph landscapes, waterfalls, sunsets etc? I am a tour guide so need just one when on my hikes. Thanks for your help in advance.

I also would love it if someone could explain why people have overlapping lens focal lengths? E.g. A 15 - 85mm and a 17 - 40mm for example. Wouldn't the range between 17 - 40 be covered with the 15 - 85mm???? I don't really get this?