If you have the old models, you can play 'count as rules' and use any of them, but here is a brief rundown without any intended bias:
9.2.1: probably the most tested yet also one that has required some update which has not happened. You will find all lists after 9.2.1 have attempted to fix the problems with 9.2.1
Leviathan: A more modern interpretation where your 'slug' tanks may have to count under some of the Carnifex options. This list has received some good feedback on initial presentation and the Army Champ (AC) is active and has played the list
Onochus: Dedicated to the old style miniatures and has started to gain some heavy playtests in the past year. There has been some great support for this list also. AC (or players) are active on the lists.
Hive Fleet Tarrasque: In the same vein as Onochus, a similar attempt to show a different list from 9.2.1 with less complication and faster game play. It is the 'new kid on the block', so no playtests to show at this time. AC is active on the lists.
Hope this helps to give a non-biased view to all the chaos

(If I have missed a list - I apologize - I am happy to update the post with any that I have missed)