Price brackets, pick the best one you can afford basically.
c. £200
Tamron 90mm f/2.8 Di Macro
(Not the VC model which is new and much more expensive)
c. £300-400
Tokina 100 f/2.8 or Canon 100mm f/2.8
(The Tokina has better build quality, but the Canon has slightly better optics and internal focusing which can be a boon)
c. £500 and up
Canon 100mm f/2.8L IS
There are arguments here for the likes of the new Sigma 105mm f/2.8 EX OS DG HSM lens which is a fantastic one, but the build of the Canon is better and there's something to be said for having a first party lens. If you want to go with Sigma then I'd look at the 150mm f/2.8 OS HSM but bear in mind on a crop camera like yours that will be a very very long lens which might not be ideal.
You can go with shorter and longer lenses but there are issues with both. Go much shorter and you lose reach and start having issues with the lens blocking light on the subject and having to get too close to things like insects. Go much longer (talking about the 180mm lenses primarily) and it becomes impossible to shoot handheld without a seriously good vibration reduction system.