The best way to get something out of Riposte Strike is probably to go against a single melee enemy with a Rapier, and make sure no one is within a shift of the enemy. Now the enemy can either attack you and get a Riposte Strike back, get an OA as he moves away or just stand there doing nothing. In in other words, you give the enemy a lose - lose - lose proposition he can't say no to. 
I don't really see the point of attacking an enemy marked by the Fighter, as the enemy will most likely attack the Fighter anyway so you will never get the Riposte Strike. In other words your attack is down to a 1[w]+dex attack - not better than a basic attack. I would rather have used Piercing Strike to be able to hit Reflex instead of AC or something like that.

I don't really see the point of attacking an enemy marked by the Fighter, as the enemy will most likely attack the Fighter anyway so you will never get the Riposte Strike. In other words your attack is down to a 1[w]+dex attack - not better than a basic attack. I would rather have used Piercing Strike to be able to hit Reflex instead of AC or something like that.