I have only played 4 and it was really fun but the bad thing about it was the game was only complete with the game changing DLCs and only half a game without a DLCs. I don't know about 5 and 6 except I know there are lots of DLCs too, so I expect the same. So, it is good this is the Complete Collection. About which one is best, I think we can decide by checking Steam votes and negative comments. Tropico 4 %91 with 7829 votes, Tropico 5 %78 with 9479 votes, Tropico 6 %87 with 17206 votes. Looks like Tropico 5 is the worst amongs the three but again I don't know what's good and what's bad about it. Have fun.