Any creature with two or more abilities is desirable, but especially those that have utility outside of being eaten by Soulflayer
The best example of this takes place in the best XYZ deck in the last five years, Purrely, as it has some of the most unique mechanics for an XYZ-focused deck that the game has ever seen
A blue Background is your best bet with Wyll since this will grant access to some other powerful dice-rolling effects like Pixie Guide
The Lightning-type Pikachu & Zekrom- GX is a powerhouse card, capable of searching up Energy and striking multiple Pokmon
The more spells you cast, the bigger of a creature you get at the end
Im taking a more holistic approach, considering a legends raw power, its impact on the format, its enduring popularity, and the uniqueness of its design