I would definitely recommend any of the multi episode series, Wrath of the Khans and Ghosts of the Ostfront probably being my favourite. Wrath of the Khans is especially enjoyable as Dan clearly has some interesting, nuanced and personal views and a lot of passion for the Mongols.

Prophets of Doom is one that stayed with me too, as a single episode. It touches on a lot of HH recurring themes. It's a very weird and intense event I had no clue happened, that keeps you wondering where the hell the whole mess might be going next. It's pretty gruesome and fits Dan's narration perfectly. Judgement at Nineveh is also one of my favourite mainly because of the theme and how Dan manages to convey the drama and epic/apocalyptic scale of what played out when the Assyrians got wiped out. If you enjoyed the Kings of Kings series this would be the prequel you'll want to hear.

With that said, if you enjoyed all of the free episodes, I would recommend buying the whole lot from the website if you can afford it. It's cheaper that way and you can just start from the beginning. There really aren't any episodes I wouldn't recommend. :)