There is no BEST powermeter, each one has its pros/cons and you must decide the one that suits you best.
I first had a polar power meter, I made the mistake of buying before reading, and found that it could have problems with indoor trainers. I also switch between cassettes frequently, and the distance between the sensor and chain is crucial, so when I put the 12-17 cassette, this space is too wide for correct readings.
There is also Power tap, but then again you have to use the same wheel for training and racing, and since I have three wheelsets (I use one for racing, one for road training and one for the indoor trainer) this option was discarded. Powertap is great if you have mora than one bike as you just have to change the wheel from one bike to another.
SRM was discarded for it's price.
And the best option for me (but remember this is personal) was ergomo, I can switch wheels and use it with my indoor trainer. The software that comes with it is great. People at ergomo are very friendly and they answer your questions almost inmediately. I have had my ergomo installed on monday and only did one ride as I am in my rest & recovery week, so I can't give you any feedback on it, if you wait two or three more weeks Ill post some opinions.
The good or bad thing about power training is that you have to read alot, and since english is not my first language and all what you read is very technical I find some difficulties in understanding it, but it's also a challenge.