I've been playing around with stats for combat for my MUD, and I think I've got it sorted out.
Here's the rundown of the average chance of hitting a character:

              |Fighter Thief Wizard Ranger Damage (Avg/Max)Fighter       | 89%     48%   47%    72%    3/5Thief         | 89%     72%   72%    80%    1/2Wizard        | 97%     89%   27%    72%    7/8Ranger(bow)   | 78%     57%   57%    70%    2/3Ranger(melee) | 57%     17%   16%    34%    2/3

The columns show how easy it is for that character type to be hit by another, and the rows show how easy it is to hit that type.
For example, a thief has a 89% chance of successfully landing a blow on a fighter, but a fighter only has a 48% chance of hitting a thief.
When a fighter hits, it does on average 3 points of damage, but when a thief hits, it does an average of only 1 point.

These are only stats for level 1 stuff (I.e. basic stats and normal weapons), and assuming that the armour can absorb a maximum of 1 point of damage. Different classes will have different armour and hit points of course, so they'll be able to take damage differently.
Now I'm going to go off and test what happens when I level up...