massive from native instruments is a bass monster!!!
but any synth that can do a good sine wave will do.....first begin with a single mono sine wave with no resonance, next increase the unison(the oscillator will produce more sine's simultaneously) value up to 2/3 (at this point you may find the sound becomes quiter!) now slightly detune your sine waves until the sound thickens up. next for movement assign an LFO to the filter cut off and play around. Add some tube warmth or somthing untill yur happy. i like to think of thick pure bass as being like a chunky rope weaved with sine waves and rolled tight!!!