Koroviev said:
Tzeentch will thrive in the hands of a player that likes to mess with other players and every card in their deck has its uses. While the warpstone travel power is strong, I don't think it's automatically the first power you should take. Old World cards can greatly effect the board. If warpstones are abundant, another power might be a better option first (I'd say that the cultist upgrade should be taken at some point before the end though).
The only upgrade that I consider automatic is Slaanesh's 2 defense cultists. It's just going to be too strong to deny, no matter your position Everyone else is situational based on old world cards and board setup. There are clear favorites but every other god has at least 2 upgrades where I'd say I'd take 1 in a certain situation and another on a different board.