The most effective braking comes from the wheel brakes. The wheel brake effectiveness is dependent on a number of factors ranging from the availability of antiskid to runway condition, brake condition, brake temperature and aircraft weight, etc. All else being equal, brakes are still the single most reliable, and most effective means of stopping the airplane.
Reverse thrust is most effective at high speed when the internal drag of the engine is combined with the faster landing speed of the aircraft to produce the most total drag. Reverse thrust is a function of internal engine drag (intake or inlet drag). The reversing action of cascades, blockers and clamshells isn't there to direct thrust forward...that's fairly minimal and of little effect. Where the true braking action comes in reverse thrust is by removing the forward thrust...all you're left with is drag. The higher the power setting the greater the drag. Net engine thrust is output thrust minus intake drag. Take away the output thrust, and you're left with drag. The higher the airspeed, the greater the effect, and the higher the power setting the greater the effect.
This still doesn't make the retarding action of referve thrust more effective or greater than wheel brakes. It also means that the actual values vary with air density, landing speed, etc, and can't always be relied upon. For this reason, landing distances are calculated without the availability of reverse thrust (in most, but not all cases). Brakes are essential, reverse is gravy.
Ground spoilers aid in braking effectiveness by spoiling left and allowing more weight on the brakes; more weight on the brakes mean more brake energy. Aerodynamic drag on spoilers is greatest at higher airspeeds.
Spoilers and reverse are more effective at high speeds than at at low speeds, but this does not mean they are the most effective components in stopping the aircraft at a high speed or low speed. Just that they're at their most effective stage at high speeds.