That effect on its own is worth around four or five mana, but the second and third chapters of the saga then go on to double the amount of counters on a creature
Opponent trying to pressure you
Usually they replace a common card to give you a random card from the set with a shiny foil treatment but again this can lead to an imbalance as my opponent might end up with 3 mythic rare cards if they get lucky with the randomiser
Wulfgar of Icewind Dale plays particularly nicely in doubling up Durnans attack trigger
Finally, Dread Summons is a card I personally don't love for requiring a lot of mana to be good and being inconsistent, and Maskwood Nexus is almost useless now that there are only two creatures in the upgraded list that aren't Zombies
However, with recent graded sales picking up, it's unlikely that Gyarados' price will drop much further, so look out for it to rise on this list in the future