Megas are Basic, Stage 1, and Stage 2 cards, and the latter two can be evolved directly from their respective Basic card or Stage 1 card
The best one-drop in history

-excessive permanent removal (clearing each opponent's board repeatedly, or Obliterate/Jokulhaups) -things that limit what you can do (like Rule of Law or Collector Ouphe) -excessively pushed Commanders in Casual (stuff where they throw Ward on for no reason, most of these are also the payoff and enabler in one card) -Chaos effects that turn the game on it's head (Thieves' Auction and Grip of Chaos for example) -stuff that prevents untapping (Static Orb and Winter Orb) -stealing everything strategies (flickering Agent of Treachery infinitely, Insurrection late game) -excessive discard (multiple Necrogen Mists and Bottomless Pit effects) -'win on the spot' effects that don't have an upkeep trigger (there is a few of these on the banlist, like Biorhythm and Coalition Victory) -counter everything strategies (Dovescape) -overly efficient tutoring (Zur the Enchanter *f-etch* fetching out Necropotence, and people are weirdly salty about Vampiric Tutor) -faster elimination strategies that encourage you to target a specific opponent (think anything Infect, and lower to the ground Voltron decks) -I think the last one I can think of is optional tax effects that encourage people to screw their pod and play into the Rhystic Study, if the tax is just a tax, like Thalia, Guardian of Thraben people tend not to mind as much, it's the ability to king make if you're dumb

Two of these holographic cards are very difficult to grade in general
Collect Vampire cards with opulent alternate art, showcase frames, and a mean bite