Everything you see in this guide may not be what is considered traditionally the best or work in every situation. Essentially this guide is just a basic format for the way I play Janna.

Ultimate Hunter doesn't do much early game like all the other Hunter Runes, because they need to be stacked. In the mid-late game UH will save you about 10 seconds on your Ult. UH doesn't reduce any more than 10 seconds late because of the way Haste scales and Janna typically builds a lot of Haste. If you don't build a lot of Haste then UH is absolutely worth using to lower your Ult CD. If you build a lot of Haste then using UH is up to your preferences. Would you rather have mid-late game MS or active CDR or 10 seconds off your ult CD? You could even go down a different Rune tree and bypass the Hunter Runes altogether.

Shurelya's
Shurelya's is best when your team lacks engage or disengage. It is also very useful for helping your team kite.

Imperial Mandate
Imperial Mandate is necessary when your team lacks damage. It also compliments burst comps.

Locket
Locket is very strong against burst champions. Works best when the assassin can 100 to 0 a carry before Moonstone can even proc once or the carry can take a step with Shurelya's.

Moonstone
Moonstone works best with tanky or poke oriented teams, so you can proc Moonstone plenty of times and it doesn't go to waste.