Here's another vote for a standard zoom, either the 24-105 or the new 24-70.
There's a reason they're called standard zooms, and that they're the lenses the manufacturers sell in the box with the camera. You'll find plenty of edge cases where a specific lens outdoes the standard zoom, but the standard zoom is going to be good to excellent at everything.
Your 50, for example, as good a lens as it unquestionably is, makes for an absolutely miserable 24mm lens. And while the 135 will be great for headshot portraits, it's going to suck royally at most everything else. A standard zoom, on the other hand, will do quite well at both wide-angle landscape work and headshot portraits, even if it's not the absolute best lens for either.
If you know specifically what you'll be shooting, grab the specific tool for that specific job. But if you don't know what you'll be shooting, you want a Swiss Army Knife that'll get the job done (and do it well even if not perfectly) no matter what. And that's what a standard zoom is.