I dont really have a bad thing to say about Tireless Tracker, but you obviously cant include it in rarity or set-restricted Cubes, and I could see it being problematic if youre into very low-power environments
As for spells with X in their cost, X can be anything (even zero), and X always counts as 0 for CMC purposes
Each character has different health, attack, bonus effects, and weakness stats, so youll want to keep an eye on those as the battle progresses in order to choose the best one for attacking during each turn
2nd place: Gengar (FO 5) 1st Edition Silver this time goes to the Ghost Pokmon Gengar
While vigilance is an evergreen mechanic, and so doesn't get specific support for decks in the same way mutate or ninjutsu might, there are still some interesting ways to use it