Average Handling Times
I think this is a very basic, but very important question which we don't think about enough. I think the answer depends on why you want to know your AHT? Couple of observations:
1: there are three common measures of "central tendency", the mean, median and mode (modal). We tend to use the mean but as you point out, if the distribution is skewed this is misleading, for instance if you have a few very long calls the mean will be higher than most of the calls. The median is better, in that it tells you the AHT in terms of the length of call where 50% are longer than that, and 50% are shorter. But you can argue that the best measure is the modal AHT, which is the most common call length (you'll need to categorise call lengths, into say 15 sec or 30 sec units to do this)
2: but if you are intersted in variations in how agents are handling calls, and how this relates to some measure of output (eg overall call quality) then perhaps the most informative measure is one of variation eg the standard deviation of call durations for each agent. Agents who are "going through the motions are likely nto have low sd's (they do the same thing irrespective of the task/customer) whereas agents who are good communicators/responsive to the customer are going to have higher sd's: their call duration will vary as a function of the customer and their task eg FCR is likely to be higher for agents with higher sd of AHT.
Hope this helps