On which side? The primary weapon for British troops was single-shot rifles, machine guns were used extensively (and to devastating effect) by the Germans but egotistical British Field Marshals bitterly opposed giving machine guns to their own troops because they considered it a "wasteful use of bullets".
Mobile armor (tanks), invented by the allies, came along towards the end of the war (sadly the military commanders once again ignored good advice and instead of deploying them en-masse as a shock weapon used them piecemeal, reducing their effectiveness); mechanized infantry (armored personnel carriers) followed soon after to transport troops quickly and - comparatively - safely across the battlefield.
Mustard gas was mainly used by the Germans, but it was dangerous for both sides because a shift in the wind could send it in the wrong direction (a brilliantly simple British invention - a fan, like a giant paper fan - was distributed to troops but some thick as s**t beaurocrat decided to save paper and remove the instructions!).
The most prominent weapon of WW1 were the "Big Guns"; cannon shells rained down day and night driving many soldiers insane and turning the battlefield into mud-soup. The Germans were largely unaffected because they built very deep bunkers (the British troops had nothing but crude trenches) and hid away until the bombardment stopped; they knew that when the guns stopped it meant that British troops were about to sent "over the top" (through minefields and razor wire under the cover of broad daylight into the waiting German machine guns) so they'd pop back up and start shooting again. One commanding officer showed some intelligence and sent his troops out BEFORE the guns stopped, only ceasing the bombardment at the last minute and so took the Germans completely by surprise... needless to say he got into a lot of trouble for showing up the incompetence of his superiors and the practice was effectively banned.