I own the Canon Rebel XSi and the following lenses:
Canon 18-55mm f/3.5-5.6 kit lens
Canon 18-200mm f/3.5-5.6
Canon 100mm f/2.8 macro lens
Sigma 30mm f/1.4
On the want list is either the 24-70 f/2.8L or 24-105 f/4L
It's really hard to choose a favorite but I'm almost leaning towards my Sigma 30mm f/1.4. I used a friend's 50mm f/1.8 (nifty fifty) but decided it wasn't really for me- I couldn't stand the 50mm for indoors and love the 30mm because it's wider. I figured if I was going to spend $100 on a lens that I'd eventually replace with the Sigma, I might as well just go for the Sigma and I LOVE it.
I'm a huge fan of macro photography so the 100mm macro is also a big favorite

. I've gotten some amazing shots with it, but it's not as versatile. If you don't love macro photography it probably wouldn't be worth it to you.
And you can't beat the 18-200mm for a great walkaround lens.
I don't use my kit lens anymore at all

I'm keeping it incase my 18-200 ever needs repairs!
I pretty much use my lenses for different situations. The one that gets the most use is the 18-200mm but I use the Sigma 30mm a lot at weddings, indoors and taking pictures of people. Macro gets used the least but I'm constantly amazed at the pictures it produces.