Direct answer: Most experts and the climbing community consider Alex Honnold’s free solo of El Capitan’s 3,000-foot Freerider (documented in the film Free Solo) to be his hardest and most consequential free solo; his later solo of El Sendero Luminoso and high-profile urban/skyscraper solos are also extremely difficult but are usually ranked below Freerider in overall technical, exposure, and sustained difficulty. Context and reasoning

  • Freerider on El Capitan is a long, sustained big-wall route with difficult technical pitches, extreme exposure, and complex route-finding, and Honnold’s rope-free ascent was widely called a milestone in climbing because of those combined factors.
  • El Sendero Luminoso (Potrero Chico) is a long, hard ropeless climb Honnold soloed and which received attention as one of his toughest single multi-pitch solos, but it’s generally considered shorter and less sustained than his El Capitan ascent.
  • More recent high-profile solos (including urban skyscraper climbs such as Taipei 101) present different challenges—endurance, unusual holds, and public/technical constraints—but they’re usually categorized separately from traditional big-wall free solos like Freerider.

Notable climbs to compare

  • Freerider, El Capitan — considered his signature hardest free solo because of length, technicality, and risk.
  • El Sendero Luminoso, Potrero Chico — a very hard multi-pitch free solo that attracted major attention and debate.
  • Taipei 101 (live skyscraper climb) — high-profile, endurance-focused urban solo with unique non-rock hazards.

Short summary
Alex Honnold’s Freerider free solo of El Capitan is widely seen as his hardest and most significant free solo, with El Sendero Luminoso and high-profile skyscraper solos notable but typically viewed as different in character or slightly less demanding overall. Bottom note
Information gathered from public reporting and documentary coverage of Honnold’s climbs.