Does riot let you choose to put a +1/+1 counter instead of haste every time or only when the creature enters?

Riot is a static ability that applies a one-time choice as the creature enters the battlefield. At that single moment, you choose: either the creature enters with a +1/+1 counter on it, or it gains haste until end of turn. You cannot revisit or change this choice later (CR 702.136a).

Once the creature is on the battlefield, the riot ability has done its job and is no longer relevant. If you chose haste, the creature loses it at the cleanup step of that turn. If you chose the +1/+1 counter, that counter stays on the creature indefinitely (unless removed by another effect), but riot itself does not keep giving you new choices on future turns.

If a creature somehow enters the battlefield multiple times (for example, it dies and is reanimated), riot triggers again for each new entry, giving you a fresh choice each time — but that is because it is a new 'enters the battlefield' event, not because riot is an ongoing or repeatable ability.

Example: You cast Frenzied Arynx (which has riot). As it enters, you choose the +1/+1 counter. It becomes a 4/3 instead of 3/2 permanently, but it does not have haste. You cannot later decide to 'swap' the counter for haste — the choice is locked in at entry.