Does riot let you choose a +1/+1 counter or haste each time a creature enters the battlefield?

Riot is a static ability that applies as the creature enters the battlefield. Each time a creature with Riot enters, its controller chooses one of two effects: the creature enters with a +1/+1 counter on it, or the creature gains haste until end of turn. This choice is made once per entry event, not continuously (CR 702.136a).

The choice is entirely up to the controller of the creature. There is no condition or prerequisite — you simply pick whichever option suits your game plan at that moment. If haste lets you attack immediately and that's more valuable, choose haste; if a permanent buff is better, choose the counter.

Importantly, if a creature with Riot somehow enters the battlefield multiple times in a turn (for example, through a blink or bounce-and-recast), you make the Riot choice fresh each time it enters. Each entry is a separate event triggering the ability independently (CR 702.136a, 614.12).

Concrete example: You cast Frenzied Arynx (a creature with Riot). It enters the battlefield and you choose haste so it can attack right away. Later in the same turn, a spell blinks it back in — you now choose again and this time pick a +1/+1 counter for long-term value.

Note that Riot only gives you one option per entry — you do NOT get both the counter and haste from a single Riot trigger unless a separate effect grants an additional Riot instance (e.g., two separate Riot abilities would each let you choose).