1.3 vanilla - great version.
1.3 cracked - great version.
1.51 - the best version of the game.
1.6 - don't use
1.61 - don't use.
1.7 vanilla - what 95% of people use.
1.7 silver edition an enhanced 1.7 version.
1.7 GOG a very good adaptation of 1.7 version.
1.71 - don't use.
1.72 - don't use.
1.73 - an okay version to try.
1.8 - don't use.
Every version since 1.6 has broken AI, since 1.7 MP is broken.
This is one case where I dislike the GOG "everything pre-patched" policy :/
Update 2013-11: there is a new patch approach by Wyrmcast over at keeperklan trying to join the best of 1.51 and 1.7.