I like Civ 4. WL is a good add-on, but I have only tried it a couple of times.
I would still be playing SMAC, but it stopped working after some Windoze patch. Also, ever since I built my latest dual core 64 bit machine, it tells me that it is not compatible.
That's easy to fix, you should just need to change a number in the ini. Change ForceOldVoxelAlgorithm=0 to ForceOldVoxelAlgorithm=1 and it should work.
I just started playing CIV 4 again a couple of nights ago. Played a fair bit of CIV 3 (bought the complete edition) and it was fine (certainly not as bad as some would make out) but wouldn't go back after playing the latest.