The most accurate of the phone apps are SPLnFFT and Sound Meter by Faber Acoustics. Sound Meter is the most accurate according to the studies I've read, but it's £50. SPLnFFT is only £3 or 4 euros as
@Rubenslash has stated, and is not far off. This is all based on using an iPhone by the way (iPhones are more reliable than android in this instance because they use the same hardware). The best part about these apps is that they can be calibrated. You can also purchase a special calibrated measurement mic (i436) which will then apparently make your phone both IEC 60651 and ANSI s1.4 compliant.
However, you can get a dedicated meter which will cost less. Make sure they state that the standards are met that I have listed above. If they do not meet these standards then they are just toys really. Because they are cheap they come precalibrated, which means they won't stay accurate forever. Measurement devices have to be sent away to be recalibrated after so long, but because they are cheap, it would probably make more sense just to replace the device.
If you want to spend more you will get better components, meaning better long term accurate results. The more expensive devices can also be calibrated to ensure results stay accurate.
The device I use is not available at the moment, but a quick search found this:
https://www.amazon.co.uk/gp/aw/d/B0...ibel+meter&dpPl=1&dpID=41lgiPeyOwL&ref=plSrch
All I was really looking for was the wording stating that it was precalibrated and met the appropriate standards. I haven't used the device above however so I can't recommend it based on first hand experience. If you search around I'm sure you can find a device that meets these requirements with maybe some reviews to give an idea of how good it is.
A good idea to test any meter is to find a device (like a hoover) that's been professionally measured and roughly gauge the accuracy of your device based on that. If it's out by a considerable amount I'd probably doubt the accuracy. Bear in mind that there is a big difference in measurements based on the surroundings (hardfloor, carpet etc) and the distance you are from the object you are measuring.
Hope this helps.