Which one is best of these?

[76561199437978235]

Gold boost + 15%
Speed step + 1
More fireflies + 3.5%

[zephyr_goldwing]

Depends what you're trying to do, your play style etc.
Speed step is good if you're running the game with an auto-clicker or playing actively, as you'll reach enemies faster, which means more money/elixir.
Fireflies are good if you're playing actively/semi-actively (e.g. game on second monitor) or have the frog equipped with at least its first or second ability unlocked (an auto-clicker can also be very helpful for firefly storms).
Gold boost is probably the go-to if you're playing idle/offline or the other two don't suit whatever achievement you may be going for (e.g. upgrade tiers without defeating any enemies or fireflies).

edit: you can reset for 150 amber, which is a nice small amount if you're trying to hit a very precise amber spend value for a holiday task.