I have played with a few pH meters in my DWC career. This will try to determine what the ‘best handheld pH meter’ is. I test VIVOSUN/whitelabel Amazon pH meters, the mid-tier Dr. Meter pH meter, Bluelabs pH meter and the Milwaukee Instruments MW100 pH meter.
Overview
Whitelabel Amazon pH meters
The whitelabel amazon style is the worst type of pH meters and should be avoided at all costs. A pH meter is a sophisticated piece of equipment with a glass probe filled with solution. These Chinese knock-offs are unreliable, inaccurate and some can’t even be calibrated. Save your money and stay away. If this is all you can afford you’re better off using pH strips and measuring the acid required to bring your tap water to pH 5.5 with a given quantity of water.

Be warned… if you buy one of these it will end up in the garbage.
As a side note: cheap Amazon EC meters work just fine. They just measure electrical conductivity which does not require a sensitive glass probe like a pH meter has.
Dr. Meter pH meter
For a long time the Dr. Meter pH meter was my go to. The company had great support and sent me a unit when mine went down. The unit costs about twice as much as a cheap pH meter but half the cost of Blue Labs/Milwaukee instruments. While I still use the meter I have I can no longer recommend it’s purchase as the company has stopped supporting the product.

They launched an updated version, but there was no real updates done to the device. I think it was to stop people from buying a new one and returning their old model.
Blue Labs pH Meter
The BlueLabs pH meter is my second favourite. It is fast, reliable and accurate. The numbers don’t jump around. It’s made in NZ and they stand behind their product. This one is really fun to use. The protective case has a silicone sleeve that keeps the electrode moist – an integral part of having a functioning pH meter.

My only complaint about the Blue Labs pH meter is the lack of a replaceable pH probe. My local hydroponics shop claims people have had units run 10+ years without issue (mind you these are meticulous users that never let the electrode run dry). At the end of the day the electrode is a consumable component and there should be the option of replacing it without having to replace the full unit.
Milwaukee Instruments pH meter

The Milewaukee Instruments pH meter solves the problem of the non-replaceable pH probe that the Blue Labs pH meter has.

Calibration is extremely simple. All you need to do is place the probe in either a pH 4/10 solution of neutral pH 7 solution. From there it’s two physical dials to tune the pH to to right value. I really prefer this physical calibration over the electronic ones like on the Dr. Meter and Blue Labs pH meter.

Perhaps the best part of this pH meter is your can stick the probe into a DWC reservoir and easily get a reading, unlike with a stick style pH meter where you have to get your head in there to see the reading.
The MW100 runs on a 9V battery. There is no option to run it on a power supply but if you’re handy you could probably rig something up and have continuous monitoring at a fraction of the price.
pH Meter Maintenance
The most important part of owning a pH meter is never letting the glass electrode dry out. The schematic below shows how handheld pH meters work.

The BlueLabs meter comes with a KCl solution they recommend keeping the tip saturated in (or dropping a few drops into the cap after use). The other pH meters recommend keeping it in a slightly acidic solution (i.e. pH 4). Never put a pH meter in distilled (R.O.) water. This 0ppm water will pull electrodes out of the glass tip damaging the meter! Same reason you don’t want to feed RO directly to your plants.
What is the best pH meter?
There is no “best” pH meter but my favourite is the Milwaukee Instruments MW100, followed closely by the BlueLabs pH meter.