You're not picking someone who sucks. You're picking someone with the precise point of having zero functional power, outside of presiding over the Senate. He sits around all day, waiting for something horrible to happen to the President.
The very point is to pick a VP who is LESSER than the main candidate. Someone who doesn't take the lime light. Someone who can perform at your level, not above it. What you describe in your potential VP pick for McCain is someone who would literally make everyone in the country who supports the Republican candidate wake the fuck up and see how horrible he is in comparison, since you apparently want him to have every strength McCain doesn't have.
So what you're really looking for is someone to fill in a mental gap, someone who makes McCain seem better by association. In this way, since McCain needs help with evangelicals in the core Republican base or make a grab for the independents, you'd either pick someone with extremely strong religious ties or cross-party appeal to back McCain's "I'm really a maverick, honest" argument.
My personal pick for him? Above we see the stir Lieberman is causing. He's actually a much stronger candidate than people think, simply because of what it represents. It'd affirm his maverick image, he wouldn't overshadow McCain (and he hasn't when accompanying John McCain already), and it'd also give the suggestion of cross-party appeal. Lieberman is not a better speaker than McCain, nor is he more charismatic... he is not a camerahog. He would be ideal.