I’m not talking about fluency, but about accent, how foreign do you sound to a native speaker?
For example, I’d say that having a perfect accent in English is really hard because of stress patterns, reduced vowels (like the schwa), which aren’t always explicit, such as in “shouldn’t have” /ˈʃʊdn̩təv bɪn/ and the distinction between short and long vowels.
By contrast, in Polish or Italian, since the spelling system is more consistent and stress is more regular (penultimate syllable in Polish, and often in Italian as well), I’d assume it’s easier to achieve a near-native accent.
I’m sure it also depends a lot on your native language. Do you think Dutch or German speakers tend to master the English accent more easily because they already have diphthongs and a short/long vowel distinction?