The question is: What sort of functionality are you wanting exactly and what are you wanting from your chat client?
Pidgin - Open source, freeware, robust support of numerous chat protocols and fairly mature at this point, multi-platform (can run it on Windows, Linux and Mac), optional integrated spell checking
Miranda - Freeware, skinnable/customizable, numerous protocols supported, growing support base
Trillian - Commercial, free version available, has the most 'finished' feel (due in no small part to its commercial nature), fairly robust support of numerous protocols, now has a web version, support for
Digsby - Supports the major chat/IM protocols, ties in with social networks, skinnable/customizable
Meebo - I'm not as familiar with this one, but does have the advantages of a web client (ie. no need to install anything, anywhere you have unfiltered web access you could have the same chat client, etc)
Universal ( to the best of my knowledge) - No third party chat/IM program works for video/audio conversations. If you're wanting to video chat with someone on MSN/Yahoo, your pretty much stuck with their individual clients (in reviewing, it appears that Trillian may actually support this, though I haven't tested such). If the primary (AIM/MSN/YIM) changes their protocols (which AIM was notorious for doing just to screw up the third party clients) your chat/IM ability may be down until the third party updates. Lack of certain other functions you may take for granted (particular emoticons/themes/pings, etc)
Pidgin - Fairly basic interface, not really customizable to as large a degree as the others, plain text password file (ie. if you have it save passwords, you can read them in the setting file), occasional issues with file transfers/slow file transfers
Miranda - Fairly new player to the market, I've seen comments regarding occasional problems with messages simply not going through (though this may be fixed in newer iterations). only available for Windows
Trillian - Commercial. Sort of bubbly interface that gives it the look/feel of almost any other Windows client, tends to be 'bigger' than other chat clients (more hard drive space, more RAM intensive, more proccy intensive).
Digsby - Doesn't support all the eclectic protocols (yet), still fairly new player to the field though growing
Meebo - As with any browser driven application, if you have filtered internet (ie. work/school) you may not be able to use your chat client, you also run a higher risk of third parties being able to intercept your conversations.
As an aside, I will say that I use Pidgin and have used it happily for several years, at home and at work. I use eBuddy on my Droid phone as it uses my data plan rather than text messaging, and for video/audio chat I use the original clients (YIM, MSN. Skype, etc.)
Hopefully, though, this at least gives you a few ideas as to what to look at with a given client and helps you to figure out what it is you want it to do for you. No single solution is best for everyone. Your mileage may vary, see detailer for deals.