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Blightsteel Colossus 8.5 For cards that can one shot people this is probably the best

I would be a little worried about a possible standard reprint, simply because it would probably would have less of an impact on the format than Scavenging Ooze, and as the reprinting of Scavenging Ooze and Hornet Queen in recent core sets shows, WoTC isn't afraid putting expensive commander cards into standard if they think they are safe

So why mono black

They put it in the envelope, take it to the post office, and ask for tracking

Some of them are typal powerhouses that care about growing an Army of the Damned while others offer unique effects