AoS I'm very ad at making decisions

- troglodon or carnosaur
--> depends on your army. If you go skink heavy or magic-heavy the Troglodon is very good. But all of the three (there are two different Carnos) have their uses. If you go for Saurus (either Knights or Warriors) the Carno is the more useful one. It is also considered to be the stronger option overall.
But if you have a Troglo, a Stega, and a Basti you can build a Thunderquake Starhost.

- engine of the gods or stegadon
--> I'd go for the Stegadon with the Skystreak Bow, except if you are a gambler because the EotG CAN be very powerful, but it is very random. Definitely go for the Stegadon if you don't intend to use a Slann or a Starseer to improve those EotG rolls.
But you can build the model in a way so you can swap between those options! So you can have both. See my paint blog for details.

- slann Mage or lord kroak
--> If there wasn't a guy here on the forums convincing me of the opposite (thanks @darren watson !) I'd say Slann any day. Kroak has only very specific uses and he is too damn expensive for smaller games (which pretty much means everything below 2000 points)

- stegadon/engine of the gods or bastilaton
--> I love the Basti and he is pretty good. One of our best units that is missing in almost none of my lists. I'll be bold and say: BOTH! Because the Stegadon's Skink Alpha can improve the Basti very well. Combined they are very cool.

 

Both, just buy them all.