Hey, I just wanted to try making a guide for Janna. I think I am pretty good with her and wanted to share some ideas. I may or may not update this depending on if I am lazy. Please feel free to post what you will on this guide.

I would focus mostly utitily, even though I have seen a few guides do a defense utility mix. Unyielding and block stop 5 damage per attack, which does not seem significant to me. The higher level benefits of utility like movement speed and cooldown reduction seem more worthwhile.

This is pretty straight forward. You need straight armor against AD carries, mana regen for sustain, magic resistance for late game, and gold to help pay for wards. You can use a quint of health or two if you are having trouble staying alive.

Heal keeps you and your team alive. An extra couple hundred health in a team fight is not bad. Exhaust is really useful to gank a carry or to shut an AD champion like Rengar down. I have seen people take flash for escapes and to use the ulti, but this is something I have not done. It might be a good idea, though.

I have heard of people staring eye of the storm, but I prefer howling gale. This can do a lot of damage early game for zoning, it can help you and your carry escape, and can help if your team decides to invade enemy jungle. From there, focus on maxing out eye of the storm. Zephyr's speed boost should be leveled 3rd in priority after your ult and shield for the speed boost. Howling gale is mostly for interrupting, not damaging. Upgrade this last in priority.

Start with faerie charm for mana 3 sight wards for safety, and mana or health potions for sustaine. First time back get the philo stone to end your mana problems and for the gold. The next trip back should be for boots, and sight stone if possible. Next, get the boots for cooldown reduction and start building your aegis unless a teammate is building it. Upgrade the philo stone to Shurelya's for the active, increased mana regen, and cheap stats. Afterwards upgrade the sightstone to gain more wards and health. Captain lets other allies keep up with you and fall back to you if the team fight is not going well. Upgrade your aegis to a runic bulwark for increased stats and aura. Finally, the last item you get is up to you. Twin shadows or shard of true ice are good choices. You might just use this slot for oracles elixer, and extra sight or vision wards because the 3 from the sighstone may not be enough.