There is no single pump that is perfect. Bottom line: no pump can mimic a baby at the breast perfectly. That said, here is what to look for:
Wall adapter is more effective than battery powered. Battery options are important for flexibility, but should not be relied on exclusively.
Wearable pumps are generally less effective. While the convenience is sometimes worth it, make sure this isn't the only pump you plan on using if you plan on pumping regularly.
Spectra and Medela pumps have been around long enough to demonstrate consistency and are commonly covered by insurance and widely available.
Flange fit is as important as the type of pump. If your pump output is in question, sometimes a different flange can improve your response for less money!