Here are the official stats on what is the most dangerous sport, based on my observations of Rugby, MX is much safer.
According to the Medibank Private Safe Sports Report 2006, the top 10 most injury-prone sports (based on patient presentations to Australian hospital emergency departments and general medical practices) are:
1. Australian Rules Football
2. Basketball
3. Netball
4. Running
5. Tennis
6. Cricket
7. Soccer
8. Aerobics
9. Rugby League
10. Rugby Union
Means nothing unless you know how many people participate and how often.
For the sake of the argument: if there's 100,000 people playing AFL every weekend, and there's only 10,000 people playing Union once per month, then AFL would actually be the safer sport.
(For the pedants: Unless AFL presented 40 times as many patients as Union [10 times as many participants, playing four times as many games = 40]).
The UK MSA did this for rallying a few years back - basically, they worked out the injury rate for rallies relative to the total number competitive kilometres travelled, and also against the number of competitors. Then they realised that it was a hell of a lot safer than the raw numbers lead them to believe.