My approach is a bit different. I just dont want to carry a big outfit, just the camera with 1 lens, that's it. So I ended up buying following with my 5D:
Tamron 18-200, very good and reasonable quality for holidays, sunny days outdoor with no specific goal in mind.
Sigma 24-70 EX F2.8; Yes, this is a bit big and heavy compared to the Tamron, but the quality is outstanding. I use this lens for indoor/low light shooting, portraits, a day in the City etc. etc. Turns out, the Quality of the pictures is so good I use it more and more. 24mm is still ok for wider angles and this lens is just sharp, fast, great depth of field control... Recommended.
--
HarryWong