These are the only barbell collars you should consider.

In the video below, some of the different types of available barbell collars are discussed and demonstrated, so you can see first hand how the various mechanisms work. Spring collars are arguably the simplest solution, though sometimes you have to apply some decent pressure to get them to slide on and off the bar. Velcro collars are another simple solution and won't get stuck coming on or off, though they might give way a little bit more than spring collars if you're dropping your barbell a lot. The other options covered are collars with fancier locking mechanisms, which provide the highest ease of use and the most secure hold of your weights, but be aware that they might come at a higher cost. These are also the barbell collars you're more likely to see in use at weightlifting competitons.