What I usually like to do with rap, is have them rap, and them rap a double track that is as close to possible as the first. Pan the second a little either right or left. Then get them to rap a third track. And on that track it should be just highlighting certain words. To either bring out certain content, or ryhtmic accentation. Pan that a little to the opposite of the double track.
No effect will ever make a rap track sound as full and thick as repeated doubling, by a good rapper.