Generally speaking most modern compression algorithms give roughly the same compression, and with regard to the number of cores that you can use at once, it is up to you to decide how many you want to use. Generally speaking (unless you are creating large archives) there is no reason to need more than one though. In addition, with multiple cores doing the compression, the bottleneck may become the hard drive. If you want to compress something to use as little space as possible, you should definitely use 7z. You can even crank up the compression settings to save even more space, although it will take longer to compress and decompress.