It was surprisingly difficult to find an architecture tour in Barcelona that was NOT operated by a touristic company. Finally, I found Barcelona Architecture Walks, which, like so many genuinely good tours, seems to rely entirely on TripAdvisor and local word of mouth.
Miguel was a fantastic guide. He has a strong point of view about the city and its architecture, and is not afraid to share his perspective with guests--a refreshing change from the cookie-cutter tours that often seem more concerned with rushing you through each site than providing accurate, interesting information. Don't underestimate the value of a tour operated by a trained, practicing architect (he also had great food recommendations).
In a city full of tourist traps, this tour receives my highest recommendation.