Flareon or moltres... Which one is still relevant?

Neither, for now. If you need a fire attacker, just evolve more higher level Eevees.

Moltre is stronger but Flareon is easy to collect the candy. It also can learn FT. It have to see your rare candies.

In the current meta neither have that man use unless it is for raid bosses. That being said, it certainly doesn't harm to have at least one high level fire Pokemon available.

Pick Moltres if too many rare candy, otherwise flareon will do good as well.

Moltres, no reasoning just saying if you have the candy.....moltres.

Why the downvotes?

Moltres, downvote.
Flareon, downvote.
Neither, downvote.
So, OP should power both?

Moltres. It does great damage with better HP.

Flareon is more relevant.
Easier to access high level, no double weakness, and FT is flexible and dodge friendly than other fire move.

Honestly, unlike the card games that I played, nothing is really that CRITICAL to power up or own. With the card games, if I wanted to use a certain strategy, I HAD to get various cards to make it all work or fit together...

Pokemon in Pokemon GO are all generic copies of one another so of the above, it doesn't really matter too much either way.

Do what's fun for you (I plan to power up a Lugia real soon since it's the only legendary I have over 90%)...

... and eventually, I plan to power up 1 of each Legendary to 30 as well just because...they are Legendary...

Moltres is tankier than Flareon so has more uses IMO for boss raids...Other than that, they are pretty similar...

Which to power up?
Zapdos 93% - Charge Beam/Zap Cannon
Zapdos 93% - Charge Beam/Thunderbolt

Neither...