Coin is strong right now for a couple of reasons.
A.) You get both effects of mana + gold from assists/kills (40g + 15% missing mana or 25g at tier 2 + 8% missing mana at tier 1)
B.) Easier to collect the gold income past laning phase for relic and spellthief you have to actively proc it while coin comes passively
Coin is balanced around the fact that.
A.) Has a worse statline 5% CDR at tier 1 while Relic and Spellthief give useful stats to help lane.
B.) The quest change was overall a net nerf since most supports that buy coin enjoy having an early level.
For Gold Gen Coin surpasses the other two by a large amount in the later portions of the game but in Lane Phase it's almost even.
Though I would like to see riot revisit coin maybe by making the coin on champ assist become a quest only reward as it adds a lot of income into the mid-late cycles of the game.
For Quest reward the items rank Relic -> Spellthief -> Coin (this used to be #2 before the previous change from a consumable that grants a skill-point to 8% movement speed for allies moving towards you)