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".