Since the only system I can use these days is either a P90 or a P83 (laptop and overdrive respectively), I use ICQ 98. It runs perfectly on 24MB ram under Win95, the only thing I don't like about it is that there are no hotkeys. You can't press CTRL-SHIFT-I to pop up ICQ or whatever... I really like that feature.
Other than that, ICQ 98 is perfect. ICQ 99b is pretty good... Some people draw the line between ICQ 99b and 2000a. After I had upgraded to 2000a from 99b, I found it kind of annoying in some ways but got used to it and started to like it. It was a bit more of a memory hog than 99b, but it's no big deal... Even on 96MB under Win2K it didn't bother me at all to keep it open all the time.
I tried 2000b once. I didn't like it at all. It didn't have any significant updates to 2000a, except maybe they took out a few things (that were good). So I definitely wouldn't use 2000b.
99b is a little tooooo laggy on P90/P83 systems, so for now it's 98.
But for best overall, I'd have to say either 99b or 2000a. 99b probably hits the sweet spot for most useful features and liteness.
Some people say AIM is better because it's not as bloated, but I have to disagree. I've ran AIM on these systems, and they lag just as much or more than ICQ 98. ICQ 2000 would be laggier than AIM, though.
All depends what your preference is.
-RSI
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