Muay Thai, western boxing, full-contact karate, san shou, judo, and even, arguably, fencing have the advantage over "real" martial arts that they are practiced as sports. Thus they are practiced at full power in situations of high stress. I would place them in a higher position than some traditional schools where competition is rare, or over the "point" type of karate competition. However, their disadvantage is that competition fighting of whatever type does not fully, realistically model self defense. "Fighting" is a relationship that occurs between (usually) two people with a certain amount of agreement as to the rules of engagement. Sometimes formallized, as in sport fighting, and other times informal as in what's considered "cheating" in a street fight or bar room brawl. There is at least always the agreement that both parties have made the decision to be there.
Self defense, on the other hand, occurs when there is an unwelcome attack of one party by the other(s). The problem for sport fighters often occurs when they aren't able to engage in the type of fight they're trained for. They have the advantage that they're used to dealing with and delivering real power, and that is good. But there are no rules in self defense, other than the laws of the land. Also no referees, time limits, equipment, space limitations, etc.
All this to say that there isn't any particular discipline you can engage in to make you invulnerable. Studying any good martial art with an open mind and open eyes to the ways things are does elsewhere is a good thing. Styles that preach dogma are worse than religions.
I personally like the Chinese internal martial arts if they're done with an open attitude. Some T'ai Chi schools can be extremely dogmatic and sectarian, but check out some of the students of Cheng Man-Ching in Taiwan.
