This has been kind of discussed.
Member, Boomer, can fill you in.
If I remember correctly, he said that the only one's that really could be more accurate are "lab grade" devices, that are not only cheap, but elusive.
Buy some calibration fluid (53mS) and keep it calibrated. Fortunately, mine (Vertex), has held it's calibration and is a solid device. I have not had to calibrate it since I bought it (about 5 months now). Others calibrate daily/weekly because their device won't hold a calibration (which is common amongst these devices).