Summon: Yojimbo and Summon: Ixion are probably the nastiest to keep around, as both of them have a Chapter I that lets you exile something, and if youre doing that every single turn, your opponents are going to be in trouble
You can also use cards like Ram Through to push Omanths damage through or equipment like Forebears Blade to give it trample Purphoros, God of the Forge & Arasta of the Endless Web There are plenty of wild things you can do with Purphoros, God of the Forge which can help whittle your opponents life totals away
Your odds are 1/32 for one of these, making them relatively common as a side effect of the hunt for more valuable cards
Its difficult to work out power level with strangers, so having a bigger list of the most powerful/problematic cards and then being able to say that three of them is level X, six is level Y, and so on, gives people a way to say, quickly and cleanly, how tuned and powerful their deck is