They are equally correct.

Undoubtedly a good question. They are both good words! Here are a couple of examples: "Italki is undoubtedly the most popular website in the world! "Doubtless, Italki will go from strength to strength". I prefer "undoubtedly" and "without doubt" to "doubtless". I'd say that a learner is more likely to use "undoubtedly" more naturally than "doubtless".