Byzantine Kitty is the best card in Splinterlands. That doesn't mean it's unbeatable or other cards can't also be very powerful. However, Byzantine Kitty should be considered the best for a few reasons.

There are many powerful monsters. Ruler of the Seas, Kron the Undying, Almo Cambio, etc. but summoners are a different beast. Their stats and abilities apply to up to 6 different monsters. A single monster doesn't interact as easily with all other monsters, except for some abilities like Inspire or Swiftness. Even in those cases, if they die, the bonus goes away. Summoners' bonuses stay for the whole game and can't be removed with Dispel.

Dragon monsters can be very powerful and unique. Only Dragon summoners have access to those. On the other hand, Dragon summoners have access to every single monster in the game. Yodin Zaku can't combine with Axemaster but Selenia Sky can.

There is no other summoner with Tank Heal. There are 2 with Triage but Tank Heal is unique. There's no summoner with True Strike. And there's only one more summoner with +2 speed.

I like to call Byzantine Kitty, Yodin Zaku, and Scarred Llama Mage "The Big Three" of Splinterlands. Llama is a bigger threat in lower leagues, but it's still pretty powerful in the higher leagues. It's just that, in the higher leagues, monsters get more powerful and mana caps are higher, making Llama + Kron and similar combinations less likely to work out. Kron also gets Last Stand at level 3, so he doesn't need Llama as much. The other 2 are great in any league. I'd say they get even better in higher leagues, especially Yodin because higher damage means more Blast damage.

There are other very good Summoners. Archmage Arius buffs every single attack stat. Valnamor and Prince Rennyn are fantastic, buffing speed, health, and magic/ranged attack, respectively. However, those 3 have abilities, which give them a bigger chance to get some game-changing combinations with future monsters. Chanseus the Great, Mimosa Nightshade, and Lir Deepswimmer are also good summoners with abilities but they're more defensive or reactive than the other 3. They can be fantastic in some rule sets. But they're not gonna be very good in most situations.

In every card game, there's power creep. Over the years, we're gonna see many monsters get more powerful. It makes sense, no one would be excited if we were getting less powerful cards as the years went by. The game would certainly die. There's nothing wrong with a Furious Chicken with bigger stats, if the rest of the monsters also got more powerful. It's still balanced. In that situation, a summoner that boosts stats also has a smaller % effect, so they will become less relevant, over the years. Still, there will always be some older cards that are very powerful. For Splinterlands, the cards with the biggest potential to be broken are summoners with abilities, especially offensive abilities. Even better if it's a Dragon! It is also unlikely we're gonna get summoners as powerful as the Untamed era legendary ones. There are more players now than before and the team has more experience designing cards. There are more people using the test servers to spot any potentially overpowered cards. Overall, the game is more mature now and broken cards are less likely to slip into the game.

Deathblow showed up in the first Chaos Legion reward cards. It's a nice ability to throw into a card that doesn't make it any more powerful, in most situations. That's very nice because you can add it to most monsters without reducing any stat or increasing the mana cost. It's a good way to fight the Last Stand ability and not just against Scarred Llama Mage.

Reflection Shield appeared in the second batch of Chaos Legion reward cards. This one is stronger than Deathblow because it has so many uses. It protects against Blast, the most important part of Yodin. But it also protects against the 3 types of damage reflection. It's one of my favorite abilities. It's a defensive ability done right.

However, there has been nothing against Byzantine Kitty. The only way to tackle it would be the True Strike. I'd like a reflection type ability for monsters attacking with True Strike but it could also be damage reduction. It would be perfect on a Dragon summoner because it could be played every time Kitty was also played. The only downside is that this summoner would be a must in the True Strike rule set. However, I don't think anything like this will happen anymore, unless they include it in the next mini-set (if it comes out before Rebellion). There won't be any new Chaos Legion cards from the core set or rewards. There are a few legendary Riftwatcher cards left but that's not the place for an ability like this to show up for the first time. Rebellion should replace Untamed in the Modern format, so there aren't many reasons to print Kitty counters there. That's why I think the chances to see something that counters Kitty are very low and once Rebellion comes out, there won't be any reasons to do it. People who don't want to face Kitty can simply play Modern and never see it in ranked games.

Byzantine Kitty is the best card because it's a Dragon summoner, has unique and proactive abilities, is very good in most situations, there's no hard counter and it's unlikely there will ever be one. It is also unlikely that we'll get a more overpowered summoner. I hope you liked reading this and please let me know if you think there's another card that you think is better.