If you are completely new to the game and wonder where to get gems you might want to check this guide out first.

First and foremost what you have to recognise is that every gem you invest is and should be a strategic decision that will influence how well you do in future duels.
Being able to correctly invest is what distinguishes good and bad f2p players (and people who only invest limited amounts of money)
In this guide i will give you an idea which cards are worth buying and why.

Generally when deciding wether or not a pack is worth investing in you only have to look at the SR and UR cards. Getting multiple copies of a Rare or Common card usually only requires a little amount of gems in comparison.
The first question you should ask yourself before investing is always:
Do I already have a deck that is able to hit the King of Games (KoG) rank?
If the answer is yes then DO NOT INVEST your gems. Save them until an important competitive event that you want to participate in happens (like a KC-Cup or a Meta Championship Series).
This way you will be able to play the best possible deck for that event.
However if the answer is no then you want to invest your gems until you can play one of these decks (or a similar list):
Tier List last updated 05.03.2018 (and is therefore outdated you can find up to date tier lists on Duel Links Meta[www.duellinksmeta.com] for example.)
Tier 1: Red-Eyes-Zombies[cdn.discordapp.com],Aliens[pbs.twimg.com]
Tier 2: Gladiator Beasts[cdn.discordapp.com], Magnets[cdn.discordapp.com], Sea Stealth Attack[imgur.com]
Tier 3:Cyber Angels[cdn.discordapp.com], Dinos[cdn.discordapp.com], Snipe Hunter Volcanics[cdn.discordapp.com],Hazy-Flame[cdn.discordapp.com]

Now im gonna go about each box in depth: (I will only focus on SR and UR cards. Some boxes might have Rare and Common cards worth getting)
Boxes you should never invest:
-Echos of Silence
-Land of the Titans
-Chaotic Compliance (this box has some good Rares and Commons)
-Wonders of the Sky
-Age of Discovery
-Sorcerer's Alliance
-Dragonic Force
-Hero Rising
-Legendary Warriors
-Flames of the Tyrant
-Electric Overload
-Destiny Rulers

None of theses boxes have UR or SR that are played in the competitive decks listed above.

The most important thing to know about a box is when to reset it.
In large boxes you only have to look at the UR cards, since you will get the SR card 6x by going through the box 3 times anyway.
In small boxes you have to take the SR into consideration aswell.

Ultimate Rising:
Decks that are supported by this box:
Cyber Angels
Cards you need to get before resetting this box:
Sonic Bird,
Good cards you might want to get before resetting:
Sphere Kuriboh
Cards that might become good in the future:
Fusion Gate

Neo-Impact:
Decks that are supported by this box:
Cyber Angels, Snipe Hunter Volcanics
Cards you need to get before resetting this box:
Senju of the Thousand Hands
Good cards you might want to get before resetting:
Mirror Wall
Cards that might become good in the future:
Stamping Destruction

Valkyrie's Rage:
Decks that are supported by this box:
HazyFlame
Cards you need to get before resetting this box:
Soul Exchange
Good cards you might want to get before resetting:
Divine Wrath (Sometimes used as a tech card)
Cards that might become good in the future:
Escape from the Dark Dimension
Dimensional Alchemist

Crimson Kingdom:
Decks that are supported by this box:
Red-Eyes-Zombie
Cards you need to get before resetting this box:
Gozuki
Good cards you might want to get before resetting:
RedyEyesWyvern(The Red eyes players are starting to cut him down to 1 or 0)
Cards that might become good in the future:
Knight of the Red Lotus, Brohunder, Dark Paladin, Reinforced Human Psychic Borg

Dawn of Destiny:
Decks that are supported by this box:
Dinos, Gladitator Beasts
Cards you need to get before resetting this box:
Super Rush Headlong
Good cards you might want to get before resetting:
None
Cards that might become good in the future:
Sacred Phoenix of Nephthys, Il Blud, Heavy Knight of the Flame, Tribute to The Doomed

Generation Next:
Decks that are supported by this box:
Hazy Flame, Gladiator Beasts, Snipe Hunter Volcanics
Cards you need to get before resetting this box:
Floodgate Traphole
Good cards you might want to get before resetting:
None
Cards that might become good in the future:
King of the Swamp (might get good if good fusions come out), De-Fusion, Evil Dragon Ananta

Servants of Kings:
Decks that are supported by this box:
Gladiator Beasts, Aliens, Magnets, Red-Eyes Zombie, Sea Stealth Attack
Cards you need to get before resetting this box:
Wall of Disruption
Good cards you might want to get before resetting:
None
Cards that might become good in the future:
Forbidden Chalice, Draining Shield

Galactic Origin:
Decks that are supported by this box:
Gladiator Beasts, Sea Stealth Attack, Cyber Angels
Cards you need to get before resetting this box:
Cosmic Cyclone, Saffira, Queen of Dragons
Good cards you might want to get before resetting:
None
Cards that might become good in the future:
Upstart Golden Ninja

Blades of Spirit
Decks that are supported by this box:
Aliens, Magnets, Dinos
Cards you need to get before resetting this box:
Powerful Rebirth
Good cards you might want to get before resetting:
Fire Formation - Gyokkou (Tech Card. You most likely need 2 max)
Cards that might become good in the future:
None

Primal Burst
Decks that are supported by this box:
Magnets, Snipe Hunter Volcanics
Cards you need to get before resetting this box:
Gaia Plate the Earth Giant
Good cards you might want to get before resetting:
Volcanic Rocket
Cards that might become good in the future:
Snoww Unlight of Dark World, Lyla Lightsworn Sorceress, Dark World Lightning

Resonance of Contrast
Decks that are supported by this box:
Snipe Hunter Volcanics, Red-Eyes Zombie
Cards you need to get before resetting this box:
Snipe Hunter
Good cards you might want to get before resetting:
None
Cards that might become good in the future:
Bujin Yamato

Abyss Encounters
(This box is very good for new players as the best deck right now is Aliens which are mostly rare cards)
Decks that are supported by this box:
Sea Stealth Attack, Aliens
Cards you need to get before resetting this box:
None
Good cards you might want to get before resetting:
Gishki Chain, Abyss Soldier, The Legendary Fisherman 2 (all are played in different version of Sea Stealth Attack)
Cards that might become good in the future:
Mystic Piper, Archfiend Emperor The First Lord of Horror

If you still have questions you can ask me on twitter https://twitter.com/Kait0o0