For serious portrait work, get primes and not because of image quality. They help you to get better because they are harder to use. You have to take time to get your photos and that makes them much better. With fast lenses as the primes suggested below you have the means to get your background nice and blurry, but don´t overdo it. Just because you can doesn´t mean you must or even should.
Skip all kit lenses or go for the 18-55 only since it´s almost free and might be nice at patries where you get beer all over your gear...
get the 50/1.4 from either canon or sigma, the 30 /1.4 sigma or the 28 /1.8 from canon (siggy if you want to stay on a aps-c body forever or canon if you think you want to move to full frame sometime in the future). The 85 /1.8 is mentioned by others here and it is a great choise, or if you want even more reach go for the 100 /2.
For portrtaits, all you need is a few primes. And some flashes. And most of all people skills. Zooms are great for when you don´t have time to interact with your subject, but even in a stressfull situation you usually have time to switch lenses if you interact with the person you are photographing, wich is a must for good portraiture. I only worry about my gear when looking online and when thinking about it. When I photograph, I never worry. I just use what I have and make the best of it, and usually I get pictures. There is no need at all to completely cover 18-250 mm. Those of you who have all those mm of range, have you ever taken a photo @173mm?
A zoom is also extremely usefull when you can not change lenses because of external interfearance like snow, sand, dust or rain.
Oh, and a boring focal length is 28 mm both on crop and FF. Also on FF 50 mm is also boring, but I have to agree that 28mm is the most boring focal length. That is actually a good thing because when you make an excelent portrait using that focal lenght, it´s not because you used some exotic gear but because you are a good photographer. That is a great challenge and we should start a group named portraits captured @50mm @ f/11 on FF or 28 mm f/11 on crop.
Good luck with your choise.
(and all this being said, I have lots of lenses by now, not to cover all focal lengths but to cover the different things I am shooting. So 28mm is covered in 3 different lenses, 35 mm in 3, 85 mm in 3, 135 in 2.... but I usually don´t bring all lenses on a shoot. It all depends on the job. For agriculture, i use mostly zooms because of the dust. For portraits I use mostly primes, even if the subject is a cow.)