Star Frontiers individually. Moreso than most modules in a series, Crash on Volturnus (by Mark Acres and Tom Moldvay, with Doug Niles), Volturnus, Planet of Mystery (Acres and Moldvay) and Starspawn of Volturnus (also Acres and Moldvay) don't stand up very well on their own. Taken together, they represent a single epic adventure that draws great inspiration from classic tales of pulp science fiction. To anyone familiar with Tom Moldvay's prior works, this should come as no surprise, as he clearly drank deeply from the well of pulp literature. It's precisely for this reason that I think the Volturnus trilogy has often been judge so harshly in some quarters. Acres and Moldvay weren't trying to present a plausible, hard SF scenario but rather a rollicking pulp romp -- and they succeeded brilliantly in my opinion.
The first module in the series, Crash on Volturnus, was included with the boxed set of Star Frontiers itself. I didn't post a cover image of it here, since it consisted of a two-sided map -- one a hex map of the titular planet and another the bridge of the Serena Dawn, a starliner on which the PCs are traveling to the newly-discovered world of Volturnus. The PCs are in the employ of a nearby planetary government to determine what happened to a prior expedition sent to the unexplored planet. Shortly after arriving in the Volturnus system, the starliner carrying the PCs is attacked by space pirates, forcing them to escape in a lifeboat and crash land on the planet below. They find the planet largely inhospitable, full of deadly creatures and hazardous terrain. Eventually, they make contact with intelligent beings -- the octopoid Ul-Mor -- who will aid them only if they complete a dangerous manhood ritual that will initiate them into their tribe.
The second module, Volturnus, Planet of Mystery, picks up where the previous one left off, with the PCs now members of the Ul-Mor tribe. The PCs learn from their hosts that others like themselves have been seen in the company of another intelligent race, a tree-dwelling one called the Kurabanda. The Ul-Mor guide the PCs to the Kurabanda lands and wish them luck in their quest. As they get closer, they discover a raging battle between the monkey-like Kurabanda and space pirates, perhaps the same space pirates who marooned them on this planet. If the PCs gain the Kurabandas trust, they learn that the prior expedition had been captured by the "demons from the sky" (i.e. space pirates) two weeks previously and were taken back to their base. A raid on the base reveals the existence of yet another intelligent race, the weird Edestekai, many of whom have been enslaved by the pirates as workers in their mines. In the process of rescuing the Edestekai, the PCs find the commander of the lost expedition, who was apparently saved by a "servant of the gods," according to the superstitious Edestekai. These servants live in an underground complex far away. Should the PCs investigate -- and why wouldn't they? -- they find a place with highly advanced technology, including robots. If successful, they learn that the place was built by one more intelligent species, the Eorna, who were the original inhabitants of Volturnus and are now nearly extinct. Unfortunately, their actions causes a signal to be transmitted that alerts the vilainous Sathar (who, it seems, warred with the Eorna in the past) of events on Volturnus, thereby setting up a final showdown in the next module.
What follows is a massive, multi-front melee brawl involving up to a dozen individuals and vehicles per side per combat. As you may recall, the Star Frontiers boxed set came with lots of cardboard counters and maps. Starspawn of Volturnus finally gives you the chance to not only use them (that was possible in prior modules, too), but to use a lot of them at once. It's hard to describe what joy this brought me as a younger person. Sure, "realists" can kvetch about the fact that there are so few units in each engagement and thus hardly reflective of a true planetary invasion, but I could have cared less. The battles were fun and they felt appropriately like snapshots in a much larger conflict.
Is the Volturnus trilogy silly? Perhaps -- but only if you're expecting something other than pulp sci-fi from these modules, which you shouldn't be. Star Frontiers was never a "serious" SF RPG, "serious" in this case meaning a deep and insightful exploration of, well, anything. It was, however, a very enjoyable game of space opera adventure that took a lot of cues from Saturday matinee serials of the '40s and '50s. That's not to say you couldn't do more with Star Frontiers than that, but I think it more than a little unfair to expect 2001: A Space Odyssey (insert obvious joke here) when the game wasn't written with "cerebral" SF in mind. Star Frontiers was inspired by the same books and movies as was Star Wars and ought to be judged on that basis rather than any other. To my mind, when Star Frontiers was good, it could be very good and the Volturnus trilogy is very good indeed.