After searching the Internet on and off for about a year before coming to La Paz to do my Most Dangerous Road Tour, I found Madness to be the most recommended.
The plus side of Madness is that they have good bikes, are readily contactable, have convenient offices - but importantly seem to offer good guides, and services with a good safety record.
However, I write to inform of OTHER things you might want to look for in a trip.
For example after finding Madness to be recommended as stated above, Madness confirmed there would be a trip leaving for us the next day. Now I am mindful of commercial realities that may give a company the "right" to cancel a trip, however madness canceled at 8:00pm the night before the trip leaving us no options.
Please be sure to try other companies and get more certainty about whether your trip will be on or not. There needs to be a better way to avoid disappointment.