Best Automatic BP Monitor

There is no "best", but before buying, please check this list of validated/recommended upper arm semi automated blood pressure monitors and only buy one of the validated/recommended ones:

Although this site also lists some recommended finger and wrist blood pressure monitors, I myself would avoid these since my own experience with a wrist blood pressure monitor from a renowned company and the literature on both are not good.

I myself have for many years used the old Omron 705 CP now version II (the Omron 705IT is technical the same without a printer for the results), and due to the price recommended the Omron M4, M5 or M6 which are easy to use for my patients.
GP's in my area use a Microlife, quite handy and compact too.

You should know how to measure blood pressure correctly: sit in a chair, have the monitor on a table next to you (not too low), position the cuff with the marked area above your brachial artery (the side of your elbow to your pinky) and the tube to the machine trailing over your pinky, stretch your arm while doing the measurement, usually the first or first two measurements are falsely high, disregard them, do repeat measurements until you have two measurements within 5% difference of each other. The mean of these two measurements is the "right" blood pressure measurement. Usually your 2nd and 3rd or 3rd and 4th measurements are OK to use.