When calculating landing distance, it's all brakes.
When determining landing distance, it's all brakes. Reverse thrust isn't taken into account. In aircraft with autobrakes, adding reverse thrust doesn't shorten the landing distance...it simply means the brakes don't get as hot.
When applying reverse thrust, especially in aircraft with autobrakes, the rate of deceleration (acceleration, actually...there's no such thing as deceleration, but I digress...) remains the same with or without reverse thrust...it's the amount of brake energy which varies.
Your question then, which system contributes what percentage, is a big variable. In some aircraft, the use of reverse thrust has a cumulative force with the brakes in slowing the aircraft, and in others, the reverse thrust simply means that the brakes are used less...but the aircraft slows down at the same rate and brakes-only.
In maximum autobrakes, max braking is applied, and any reverse thrust applied is cumulative...it's effect on stopping the aircraft is additive to the brakes.
The ability of the brakes to work also depends in turn on other factors, such as the dumping of lift on touchdown...which may involve spoilers, autospoilers, retracting leading edge devices, etc. Spoilers don't serve to provide much of a percentage of slowing the aircraft down, but they do serve to kill lift, put more weight on the wheels, and thus make the brakes more effective.
A better way to look at the question might be to consider the penalties if a particular system isn't functioning. A reverser which is locked out, for example, will have a landing (and sometimes a takeoff) performance penalty. This isn't really the percentage of it's contributition that's missing, but an overall effect on performance when a system is partially or wholly compromised. A failed spoiler, for example, may have a performance penalty applied which reduces the capability of the aircraft to stop or which requires some assymetrical braking, thust reducing brake effectiveness and increasing ground roll. A failed anti-skid transducer may invalidate a single wheel or a pair of wheels, increasing ground roll during a rejected takeoff or during a landing.
For aircraft with autobrakes, typically the aircraft is landed with the system armed, and when ground logic tells the airplane it's actually on the ground (squat switches, sense switches in the antiskid system, weight on wheels, landing gear truck tilt, etc), then the system autobrakes engage at a preset value. In some airplanes, reverse thrust will be applied before the nosewheel touches down, and in others it must be on the ground before reverse thrust is applied. Reverse thrust is most effective at higher speeds, as are aerodynamic braking effects such as speed brakes and ground spoilers, and the amount any particular system is able to contribute to slowing the airplane varies with the configuration, the landing surface, the particular phase of the landing (high speed vs. low, for example), and so forth. The value isn't the same throughout the landing roll so far as what system is contributing what percentage to the slowing of the aircraft. It varies.