there are many threads on this topic but it all depends on what you want to get out of the program: lounge access, upgrades, free trips . . .

I presume you have checked out the comparison chart:

http://members.shaw.ca/deercroft/starall04.html

Once you decide what benefits you want, it is a lot easier to settle on a program. It is usually though not always best to register in the program of the airline that you will be flying on most frequently if you want to get upgrades for example. If all you want is lounge access, then the cheapest way (e.g. 35k qualifying miles on all Star carriers for AC Aeroplan) to Star Gold might be the answer.