You're doing something wrong, probably don't need the "best" grease gun. Someone doing a lot of greasing, multiple times every day, may need one that's heavy duty to last longer, but that is different than a basic, fit-for-purpose problem. It is not rocket science, either the grease used was junk, or the storage environment too hot. Worst comes to worst, any non-defective gun should work fine if stored upside down in a bucket so it stays primed. Granted, this is within the context of using NLGI #2, if your grease is more viscous, you need a higher quality gun to pump it.
If your gun is defective, sure then replace it, but this has nothing to do with "best". A grease gun is a simple tool that just has to work, where paying extra is more about features if you need something specific. At the same time, don't buy the cheapest generic on Amazon. Like anything, there is a sane middle ground and then there are idealized internet posts which fail to mention that the average grease gun works fine without issues for many
years. It does not need to be "best" at all.
You do get what you pay for...
This is not true. The average economy grease gun works fine. For only occasional use, with no special requirements, paying over $30 (regular price) or $20 sale price, is a waste of money.