On one level, I will say yes, I think tai chi is the best. Obviously I do, as does anyone who spends hours every week practicing it. If I thought otherwise, I would be off learning karate or kung fu. No doy. (Although I do not practice solely for self defense...any martial artist will tell you that is not the only or even the primary benefit of martial arts training.)
In reality, whether tai chi is "the best martial art" is really a weird kind of statement if you think about it. Martial arts are not what does the fighting should you sign up for a competition or be unlucky enough to get into a confrontation on the street. People do the fighting, and people come in all formats: well trained, with tons of experience; untrained, with tons of experience; well trained, with no real fighting experience; belligerent windbags who won't throw a punch; silent but deadly types who will literally stab you in the back. Not to mention the vast majority who just won't bother getting into any type of fight.
What I can say for sure is that tai chi is an amazing way to learn how to deal with the energy of others, including, when you're ready, physical confrontation. If you want to see what it looks like when a guy with a lot of tai chi training demonstrates his skills against a mixed martial artist, go straight to the video below, featuring a tai chi demo and commentary by Ian Sinclair of Sinclair Martial Arts in Orillia.
If you want to know what really makes someone the best, though, listen to the audio commentary Ian Sinclair has laid over the video. You might want to especially prick up your ears at 3 minutes. Here I'll help you with a little transcription:
In order to master martial arts, you need to master yourself. This is not just an idealistic principle. This is a very practical thing. Every time there is a thought there is an emotion. Every time there is an emotion there is a physical response. So people who master martial arts have to work through their own stuff. There is no room for fear, hatred, anger, ego or anything on the floor.... The better the martial arts school - the nicer the people. The higher the level of skill - the nicer the personalities of the instructors.
You get this, right? Not: the higher the level of skill, the faster they'll wipe the floor with you. Not: the higher the level of skill, the more they'll boast about their level of skill. Not: the higher the level of skill, the less they'll have the time of day for you when you walk in the door to check out the class. Niceness is a good metric because anyone who has learned well should have learned mastery over themselves and therefore should be able to help you feel accepted and comfortable from your first day. That really is the best.