Fortunately I gathered much experience over the years. The most important with theese is how they are designed. You should never, i mean NEVER buy shady power supply from unknown chinesse sellers / ebay / aliexpress etc.! Majority of them are really badly designed. They might kill not only the device because of poor design, but also YOU because of poor high and low voltage separation. But the shady sellers don't really care, the job is done when the sale is made. Problems they usually have:
- Insufficient high and low voltage separation. 220V lines are too very close to 5V on PCB which can cause 220V to get to the 5V line and you are dead.
- Voltage not 5V which can kill your device. Often, when they are under load voltage starts to drop, or they emit excessive heat.
- There is a lot of noise on the output. Voltages goes up and down from like 4V to 6V or even more. This can kill your device.
So a general advice would be to buy it from respectable manufacturer like samsungs, htc, lg, apple, and other major brands. According to tests they are generally very good.
As stated in previous comment, IKEA makes very decent USB power supply (tested). Also (you wont belive) their LADDA rechargeable AA and AAA are most likely Panasonic eneloop PRO acording to tests. So IKEA is probably a safe and good choise for power supply.
I would suggest you choose based on information on this web site, where this Danish guy makes reviews of chargers with proper tests and measurements from electrical point of view:
Example of good ikea charger:
Example of BAD and dangerous fake apple charger:
Also dont watch the hipsters chargers reviews on youtube, where they comment, color of cable, number of ports, etc. it is useless. You need measurements like this Danish guy does it.