Multitools do a lot of things - cut tape or twine, tighten or remove a screw, and get a firm grip on a small object with a pair of needle-nose pliers, and much else.

Good multutools have quality build, great steel blades that can hold an edge, and and strong hinges that hold their position but are easy enough to open and shut.

Good multitools are comfortable to carry with you. The plier tips of a good multitool meet at a precise point. 

Finally, good multitools are small and compact, giving your more functionalities in as small size as possible.

Leatherman, Gerber, Victorinox (the Swiss Army knife company) are some of the bug players in the multitool market.

The best multitool: Leatherman New Wave ( great ergonomics, solid made, gerat price)

This great muititool contains:

needle-nose pliers
regular pliers
wire cutters
hard-wire cutters
a 2.9-inch 420HC knife (HC stands for “high carbon,” which means the knife will hold an edge better)
a 420HC serrated knife
a saw; spring-action scissors
a wood and metal file
a diamond-coated file
a large bit driver that flips between a flat and Phillips head
a small bit driver with an eyeglass screwdriver
a medium fixed-blade flat screwdriver
an 8-inch/19-cm ruler
a bottle opener
a can opener
and a wire stripper.

So, you get an idea of what a good multitool contains.

The second best multitool: Leatherman Skeletool (very small, only pliers / knife / small screwdriver)

The best luxury multitool: Victorinox Swiss Army Swisstool Spirit X (more smooth and polish than top choice)

The best keychain tool: Swiss Army Keychain (little knife blade, tweezers, eyeglass screwdriver, scissors and more)

The second best keychain tool: Leatherman Keychain Tool (knife blade, scissors, a file, screwdrivers, a bottle opener, pliers withtiny wire stripper)