Thalias Lieutenant demonstrates the power of putting +1/+1 counters on your creatures in Standard
This is the second best ex, IMO, behind Dragonite
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If you're the type who needs your commander in full-art foil or you're hunting serialised chase cards, Collector Boosters justify the 34 price premium
Keep in mind that pack points are accumulated separately by expansion, so open booster packs from Genetic Apex (A1) to get pack points for Mewtwo
While this might be a case of the Pokemon's limited exposure thanks to being locked behind DLC, it's going to take some time and likely an exceptionally strong, playable card for it to appeal to a wider TCG audience