Leatherman, Victorinox and SOG are the most respected of the main brands. Gerber also make good multitools, but are perhaps less well respected. Leatherman multitools come with a limited 25 year warranty. The other brands give limited lifetime warranties on at least some of their products. This doesn't mean Leatherman tools are inferior, however, and the utility of a lifetime guarantee (which will only cover manufacturing defects) is debatable. Multitools from any of these brands will be very high quality. The differences tend to be in the style and functionality.
Victorinox predominantly make swiss army knives rather than butterfly-style multitools. They are more suitable for general use and outdoor activities like camping and hiking. You may also see swiss army knives by Wenger, which are also good quality. However, Wenger was bought by Victorinox and the brand no longer produces multitools. For this reason Wenger multitools are not featured on this site.
Leatherman and SOG make mainly butterfly-style multitools that have pliers and wire-cutters. They are heavy-duty tools and very sturdy and long-lasting and suitable for many applications, e.g. DIY, automotive use, gun maintenance etc.
Gerber are somewhere between Victorinox and the other brands, and make multitools more for recreational use like camping and climbing.
Some SOG multitools have customization options, i.e. you can change the tools to suit your needs. It may also be possible to disassemble multitools from other brands like Leatherman and interchange the blades, however they are not designed for this so may not work safely or effectively and doing so might invalidate the warranty. It is generally not possible to disassemble Victorinox swiss army knives.
Multitools from these brands will be considerably more expensive than brands like Stanley, Draper and BoyzToyz, and the sort of multitools that are often sold in general stores, but will be much more effective and long-lasting.
For bicycle multitools Lezyne, Park and Topeak are well respected and make good tools.