let's go over a few critical items that ezreal needs to get in order to stay relevant:
this is a ridiculously important item in a meta where tanks are a serious threat. because your q applies on-hit effects, it will apply the passive from blade of the ruined king to its damage and allow it to do noticeable damage to the enemy frontline. it also increases your kiting potential because of its movement speed-stealing active
having a fully-stacked manamune gives ezreal his most significant powerspike and allows him to do sizable damage to the enemy team throughout all stages of the game. it's recommended that you buy your tear of the goddess as early as possible so that you can begin to stack it so that it's completely powered up by the time midgame comes out. the bonus damage from the toggle is also applied to your q, further boosting its damage
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pick one, not both. if their frontline is getting to you in most fights, grab an iceborn gauntlet. this will increase your kiting potential and allow you to peel yourself off of sticky tanks -- and sometimes assassins as well. the cooldown reduction, armor and extra mana is a nice bonus that synergizes well with your kit, too.
for all other situations, triforce is a great item that boosts every stat ezreal cares about. the rage passive from trinity force is also a great way to keep your distance from dangerous threats or catch up to fleeing enemies. picking the appropriate item is situational and largely based on preference.
cooldown reduction on an AD-based caster. simple.
you need armor penetration to stay relevant