"Best" can be a lot of things... Price, speed, capacity, ect. Obviously you're going to have to shop for one that will fit your connector, and fit in the physical space in your laptop. What type of connection does your mechanical HDD use? I'll hazard a guess and say it's a regular SATA connection with the mSATA slot being for an additional drive. In which case you could get a small mSATA drive and use it as a boot drive, and have your mechanical HDD for mass storage. Or you could straight out replace your existing HDD with a hybrid drive like the Seagate Momentus XT which has a 8Gb SSD built into it as a cache drive. Slower than a pure SSD, but faster than a regular HDD. Or lastly you could replace your mechanical HDD with an SSD, capacity being dependent on the size of your wallet.