Well your choices for a good macro lens are:
Canon EFs 60mm macro
Canon 100mm macro
Tamron 90mm macro
Sigma 70mm, 105mm, 150mm, 180mm macro
All of the above are quality lenses and will deliver good sharp results - so getting any is a good move. The general rule is that you want to start off with a lens of around 100mm or longer to give you a decent distance from the camera to the insect (provided you are shooting bugs) so as not to spook the insect.
After that consider that a longer focal length also helps with increaed background blur in a shot - often a desirable feature in macro shots.
As for my advise as to which to choose its tricky - the Canon 100mm is certainly a quality lens and the workhorse of many a macro photography - bare in mind though that it ships without a lens hood or tripod collar and that these two (IMO essentail) components need to be bought separatly. The hood is important for blocking out unwanted light and also allowing defence against things hitting the front element of the lens - the collar helps a lot when shooting from a tripod with weight distribution.
With both of those parts added to the price the cost of the 100mm raises to be almost the same as the sigma 150mm (which has both hood and collar included in the price)
The sigma 150 and 180mm models are the top macros of the line for sigma - and well worth a look. Both (like the canon 100mm) are internally focusing (which means that physical lens length remains the same no matter what you set the focus to) and further they are also both compatabile with sigma teleconverters - I often find myself using a 1.4 teleconverter to get a little bit of extra magnification which is great when working with small bugs - a 2* teleconverter is a harder combo to use and is better suited to tripod shooting.
Also the sigma 180mm is generally considered a to be a bit heavy for continued handheld use - whilst all the rest are fine for handheld work.
Note also that whilst the EF-S 60mm is very fine lens it is crop camera body only - if you want ot move to a fullframe body later this lens will not work with such a body
Hope that helps