Calorie-free beverage options

[miajag]

So lately I've been trying pretty hard to eliminate drinking stuff that contains calories (other than milk/oj with breakfast and the occasional scotch/bourbon bender). I'm wondering what the relative advantages/disadvantages to the various zero-calorie (or negligible-calorie) beverage options are. Unless I'm missing something big, there are four major ones:

- Water
- Diet soda
- Unsweetened tea (or sweetened with Equal or Splenda or something)
- Diet drink mixes like Crystal Light, etc.

Now, I'm aware water is almost certainly the "best" of these, but exactly how much better is a glass of water than a Coke Zero or whatever? Also, I'm at least somewhat aware of the issues with caffeine/lack thereof with regard to hydration. Mostly I'm just wondering if there are any of these I should particularly avoid because they're bad for whatever reason.