I recently spent some time thinking about a light sensitive lens myself, just for strong bokeh portraits and low light photography in normally lit houses, lets say on family meetings etc. I dig the 135mm 2.0 very much because of its great magical bokeh, but 135mm was too long for my taste so it had to go.
If the 135mm is too long for you, I assume you talk about 50 or 85mm then.
On 50mm you could go with the Canon EF 1.8, the Canon EF 1.4 or the Sigma EF 1.4. I would not recommend the Canon 1.8 as it is one of Canon's cheapest glasses with many problems concerning focussing. That leaves Canon and Sigma 1.4. The Sigma has the better image quality at 1.4 but causes a shitload of problems on full frame cameras, I read tons of reports about it's focussing going completely berzerk so you basically can strike that out which leaves only the Canon EF 1.4. While it is without any means a great lens from 2.0 upwards, I personally think it's quality at 1.4 is only average. It isn't really that sharp. Compare to the Sigma here
As there is no lens that met my expectations and Canon didn't reboot the 50mm as I was hoping, I drove the thought of 50mm away.
That leaves the wide aperture 85mm lenses, in fact in your price range only the Canon 1.8 or the Sigma 1.4. In that case, Sigma's got the slower autofocus but the better image quality, at a much higher price tag. You can compare Canon at 1.8 and Sigma at 1.4 here
and see that Sigma performs better on 1.4 than Canon on 1.8.
For me, I chose the Canon 85mm 1.8 because it is simply only a lens I would use between 1.8 and 2.8, because for 2.8 and upwards I got Tamron 24-70 and Canon 100L, especially the latter is hard to beat in image quality means. And for that seldom using, it was not worth the extra cost of the Sigma one.
Oh well, upgrading is always a possibility
And, review claim that the Tamron is better than the Canon 24-70 MKI. But do I really notice when not pixelpeeping?
Your money bag certainly does. Also, Tamron 24-70 got a image stabilizer which renders it unrivaled, especially at that price tag. It truly is a great lens with an exceptional build and image quality. My friend had a slight focussing problem with VC turned on, but it got exchanged without problems and now his also sits perfectly.
On the other hand, with a weight of over 600 grams, it is not what you would call lightweight, it is a heavy toy.