Basically two mates of mine have formed a rap duo (piss take) and I offered to do the music for the song they wrote in law (about an ugly emo). Can anyone recommend and VST plugin synths I can download that are raptastic?
I've driven 102,888 extra miles for every 1 mile an environmentalist has walked....
First of all: anything goes as long as you think it sounds good. Don't hesitate to try the VSTi's you already have.

Also, synths aren't really that prominent in most rap. They're used but there isn't one particular synth that's associated with rap as far as i'm aware.

That's because a lot of rap is sample based, and some of the most common devices used in rap are samplers - devices that can record sounds and attach them to one of more keys on a keyboard, used to reproduce live instruments but also to take segments from other music and mash it up a bit. For example if you have a drum loop, you can chop it up into segments so you can trigger each segment independently, that way you can add some variation in an otherwise repetetive loop. Or you can just give each drum sound its own key and record a rhythm one sound at a time. And sometimes lowering the decay of each slice gives funky results. You can even change the pitch of one slice to avoid copyright infringement Whatever, there are many many possibilities.

The Akai MPC series are what many of the pros use but there are great VSTi samplers like Battery and Halion. You'll also need a basic sound editor to cut loops, and Propellerhead ReCycle is a great tool for chopping up loops - especially when combined with the Dr. Rex in Reason. And of course you'll need source material.
As LKW said there isnt a particular synth.
got an MPC for christmas and it is amazing would seriously recommend it for raap/hiphop/dance.
Try finding a demo of ableton live. Its like the Mpc but on your computor instead.
now extra flamey
If I wanted to make a song, I would follow these three steps:

1) Get a piano sample, and loop it.
2) Record a bass line.
3) Add dialogue from an old Kung-Fu movie.

Instant Wu-Tang.