Why Agents are Still Good and will be even Better

Agents were first released in Ancient Sanctuary - they were almost entirely useless, with Venus bringing out nearly-pointless monsters, Saturn dealing a lot of life point damage but only if you were winning, Mars being big but only if Sanctuary was out, and Mercury letting you draw a card if you had no cards in hand. All in all, a flop.

Konami rectified this with a Structure Deck that brought not two, but three major cards:
Master Hyperion
The Agent of Mystery - Earth
The Agent of Miracles - Jupiter
The first two were great for obvious reasons - Hyperion is a massive monster with awesome destructive abilities and gave you a way to win games. Even better, it has no summoning conditions, so you can still Tribute Summon it or use Monster Reborn. Earth would let you search for other Agents (namely Venus) and was a Tuner. One other development made Agents good - Xyz Monsters. With Venus, you could now get a Gachi Gachi Gantetsu for nothing more than 1000 Life Points - putting Agents on the competitive map.

In addition to that, Honest and Archlord Kristya. Honest is good for obvious reasons, as is Archlord Kristya - Honest lets any of your LIGHT monsters get over anything, no matter how big, eliminating the biggest threats and protecting you for the turn if it's your opponent's Battle Phase. Kristya, on the other hand, once summoned, lets you recover a Fairy as well as stop Special Summons and just keeps coming back. Also, like Hyperion, it has no summoning conditions, so you can use Monster Reborn or Tribute Summon it.

Lastly is Jupiter. This card is normally underrated - and for seemingly good reason: it's little more than a beatstick. However, it does a few things which will be even more important when Lightray Sorcerer is released. First, it gives you some Graveyard control, giving you more opportunities to summon Archlord Kristya. Second, it gives any Agent an 800 ATK point boost - this can let your weaker Agents handle Level 3-5 monsters or allow Jupiter to get over anything with up to 2600 ATK, be is Stardust Dragon or Evolzar Laggia. In addition, once Lightray Sorcerer is released, it will be essential for creating the proper summoning conditions.

Onto the builds:
There are many different Agent Decks. Some work, others don't. Some worked last format, but do not work any longer.

First are Sanctuary builds. These use Sanctuary in the Sky in order to give Hyperion, Jupiter, and Earth their secondary effects, all of which are quite powerful - Earth is able to search for Master Hyperion, Jupiter lets you get back Banished Agents, and Hyperion can use its effect not once, but twice in a turn. In addition to that, it gives you access to the powerful Divine Punishment, allowing you to run four Solemn Judgments, three of which have no cost (and instead of negating Summons, they negate Effects, which is often even more important).
 This seems like an amazing strategy, but there must be some catch, otherwise people would be using it. This is the catch: inconsistency. Last format, these Agent builds underperformed due to their inherent inconsistencies. This format, they are useless against the meta decks because of these inconsistencies.

Next is Herald Agents. These use Herald of Perfection in conjunction with standard Agents.

Herald of Perfection is a great card, but the Deck still lacks consistency. Which means that Herald Agents are out.

There's also Dimensional builds that use Miraculous Descent but these are easier to stop, although quite powerful when the strategy gets rolling.

So, why can Agents work in the metagame?

There's a few reasons, among them:

Rabbits and Wind-Ups are Tier 1, along with Inzektors. Inzektors are just plain annoying but they're easy to side against. Moving on.

Rabbits have a few crucial, exploitable weaknesses. First is having high ATK. Jupiter will get over a Laggia as if it wasn't there, forcing them to waste backrow or Xyz Materials on it. A Hyperion or Kristya will do so as well, as will a Dark Mist. Most people thought that Gachi would be better, but Black Mist is superior in the Rabbit matchup. Not only is it able to kill three monsters, but it keeps half of their ATK, ending up with over 2500 ATK in many cases. If your opponent doesn't predict the Dark Mist, then they'll be forced to waste Xyz Materials on it, and if they do, they'll have to waste them on Venus - which leaves you open to Special Summon Hyperion.

As for Wind-Ups, Agents have amazing topdecking ability. Drawing a Venus gets them a Gachi Gachi or Dark Mist. Drawing a Hyperion gets them free advantage any day. Drawing a Kristya with 4 Fairies in their Graveyard is one of the best things that can happen. Drawing a Black Luster Soldier - Envoy of the Beginning at any time can be game-winning. And drawing a Lightray Sorcerer will allow you to get over a lot of bad situations (as it can Banish Zenmaines). From experience, I have been able to win situations after having all monsters in my hand discarded (thank you overpowered Gravekeeper support).

All in all, the Deck is fantastic. It has few bad matchups right now and these weaknesses are easy to overcome with side decking.