Today's Rap utilizes pretty much any type of synth sounds. It just kind of depends on what sound your going for.
A lot of synths that are specialized or catered for EDM (Electronic Dance Music) can be used for Hiphop as well these days.
Pretty much any synth that can make a Trance sawtooth lead or something like a monophonic moog style lead or bass.
The best IMO are: Minimonsta, V-Station, Vanguard, Surge, Poly Ana, Sylenth1, Gladiator, Discovery Pro, Synth1, Z3ta+, Pro 53....