Recently just got done with Portrait of ruin and Harmony of despair. I'm planning to get more Castlevania games in a couple of minutes at Gamestop. I only have 20 dollars on me. I only have enough money to buy one Castlevania game. Which one I should get?

Either Dracula X Chronicles or Dawn of Sorrow(I haven't played DOS yet,but if its anything like Aria of Sorrow it should be great),Dracula X Chronicles even includes Symphony of the Night and the original PC-engine Dracula X which is nice. 

 

I like Lords of Shadow for what it is,its a good beat'em up game with an interesting story,but its not really a Castlevania games,its more like God of War,so you might still enjoy it,but don't expect a Castlevania game and I wouldn't recommend you Mirror of Fate at all,its really not a fun game in my opinion.

I personally love Dawn of Sorrow, it's soul collecting thing is a great system. But Dracula X does have Symphony of the Night, which a lot of the fans would call the best Castlevania. 

Well considering you said "in a couple of minutes" you've probably gotten the game you wanted, but I'm going to give my opinion either way.

 

If you liked those two you're probably going to like Down of Sorrow (it's a sequel to Aria of Sorrow). I don't really like all that 2.5D  stuff that Dracula X Chronicles and Mirror of Fate is, but as Isaix said Dracula X has in it arguably the best Castlevania game wich is Symphony of the Night, and of course a port of the original Rondo of Blood.

 

Also I think you should stay away from all the LoS stuff until you've gotten your fill from the original series.