There is no 1 lens to rule it all (big big aperture, long long range....) yet. You goto figure which are your needs (maybe prioritise, MOST impt, Lesser impt, Least impt etc). And I think this will come up sooner or later, but, the beauty of dslr is to allow you multiple lens changes for different needs and occasions. So, which are your needs? (550D is a crop body, just a note that the 24mm can be somewhat tight in some confined, indoor areas, unless you pair with a wider lens (see what I mean by variety?)
Just compare this post with your initial one. There is hardly a link. Seems you should ponder a bit more what you are actually doing and then make a sound decision. So far, nothing much others can advise you. And: L lenses are not the only choice (unless you have unlimited budget. But even then there other other [new] choices :bsmilie: ).
IMO, like what edutilos said. You will need to think about what you really want rather than throwing $ on lens you will not fully utilize.
Low Light Event Shooting - You need either Flash or a F1.X lens or BOTH rather than L lens (IIRC, L = Good IQ and may not equals to Fast).
From my standpoint, it seems like you want to find a "all in one PERFECT" lens that is L and Fast lens and yet cover all range from 17mm to 135mm and beyond. If such lens exists, most photogs will be bringing 1 lens around than a big bag with lens from macro to u.wide to long range.
My point: If you have to change lens for a certain event/shoot, just change it....If not, just accept the limitation and move on. *no offence*
I agree with the above. However, if you really want an answer, it will be the 24-70mm, since it's f2.8. I've used before the 24-105mm, and I'm happy with that lens as you can just carry your 550D with that lens for general usage. Need not change lens. If you shoot night shots, you just need to up ISO a little to compensate for the f4 available on the 24-105mm.
Having to good / right equipment is one thing, the photographer behind the camera is another.