The Eldritch Knight fighter's War Magic feature (PHB, pg. 75) states:
Beginning at 7th level, when you use your action to cast a cantrip, you can make one weapon attack as a bonus action.
(The 18th-level Improved War Magic feature works the same way, but allows any spell as your action rather than being limited to cantrips.)
The description of Counterspell states:
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.
For example, say my Eldritch Knight attempts to cast booming blade. If someone casts counterspell on my booming blade, am I still able to make the bonus-action weapon attack?
Best Answer
Since you used the action, you still make the attack
The ability specifically says that when you use your action to cast the spell, you get to use the BA attack. Counter spell is cast in reaction to a spell being cast and causes it to have no effect. Since counterspell doesn't cause the casting to fail, simply the spell being cast, you still used your action to cast a spell. As such, according to the ability, you'd still get your attack