If you have a limited budget…
I would suggest looking for a good wideangle lens… 17mm-35mm is very usefull [I have one and it is the only lens I use – most of the time]
Wideangle lenses are excellent for landscapes since they distort perspective a little bit [but avoids the extremes of a “fisheye” lens] In a landscape these lenses creates an effect of greater depth thus making the scene appear even “grander” than normal. The 35mm end is excellent for portraits and general photography.
The Canon EF-S 17-55mm f/2.8 IS USM / PRICE: from $1000AUS+
The f2.8 aperture will give you plenty of low light capability as well as lovely soft background [great for portraits]. The 17mm end is great for landscapes too. A very useable lens.
EF 16-35mm f/2.8L II USM Lens / PRICE: from $1500AUS+
EF 17-40mm f/4L USM Lens / PRICE: from $800AUS+
These are about half the price of the 16-35, and the reason is the f4 aperture size. You wont get the soft backgrounds that you will with the 16-35 but it is still a damn fine lens.
You can also go with lenses from lens manufacturers like SIgma or Tamron which will generally be cheaper, but still top quality [I use Sigma].
The Sigma 10-20mm F4-5.6 EX DC HSM / PRICE: from $700AUS+
This is the APC Format version of my 17-35mm, I love this lens to pieces!