Choose cards that align with your deck's spell distribution and mana requirements
But at the end of the day, it's always going to have value because this is a game about creatures attacking and blocking, and this will do so very nicely, either as a 4/4 flying haste-ed one, or just a 5/5 flying that can also shoot stuff if properly en-countered (say, by putting the counter on and not immediately swinging)
Magic Arena is designed to be a fun, convenient, and user-friendly way to engage with Magic
You draw X cards and you lose X life, where X is the number of counters on that creature
Nerd Rage slots into a bunch of decks that want to spellsling and cantrip
Without flash Hollowhenge Overlord would probably die right away, but sneaking it in on an opponent's end step makes it that much more likely you'll get at least one trigger from it