Round 1: Getting in
Draw
Croatia: an ID card is enough. Croatia joined the Schengen area on 1 January 2023; Portuguese citizens enter and stay up to three months on a valid citizen card or passport. Montenegro: an ID card is enough. EU citizens may enter on a citizen card or a passport for stays under 90 days; the document must be valid three months beyond the exit date; register with the police within 24 hours unless a hotel does it. An ID card is enough on both sides, so the rule calls it a draw. The difference is that Montenegro is outside Schengen: you clear a real border, and the 24-hour police registration rule applies unless a hotel handles it.
Rule: Fewer hoops wins: ID card, then visa-free, then on-arrival permit, then e-visa, then consular visa.