There are loads of decent portrait lenses out there it just comes down to what suits your style and budget. Ideally you want a fast lens (small f number) as this will allow you to blur background nicely. The cheapest option that will give good results is the Canon 50 f1.8 (often referred to as the nifty fifty) and ~£80 it is one of the best value lenses around. But it's a fixed focal length which may or may not suit. My main portrait lens is an old Canon 28-70 f2.8 (now replaced with the 24-70 f2.8), it's a lovely lens but is big, heavy and expensive. Sigma and Tamron both make lenses of a similar spec which are a deal cheaper but I have no personal expereince so can't comment on the quality.
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