If your native language is English
Start with Spanish. It has the largest learner community, the most resources, and shares thousands of cognates with English — words like *hospital*, *animal*, and *important* are identical or near-identical. You'll find words in the grid faster early on, which makes the experience more rewarding from the start.
French and Italian are also excellent choices. Both share Latin roots with English and have very regular vocabulary patterns.
If you already know one Romance language
Try another one. If you speak Spanish, Portuguese will feel immediately familiar. If you speak French, Italian will click quickly. Passage is a great tool for leveraging existing knowledge to accelerate into a related language.
If you want a challenge
German has longer compound words and a different grammar structure from English, but its vocabulary is highly logical. Polish and Turkish are genuinely different in structure and will stretch you further.
A tip for any language
Set your hint language to the language you're learning, not English. Seeing a Spanish definition for a Spanish word forces you to process meaning in context — which is how you actually learn to think in a language rather than translate through it.