There's no synth for all, but every synth in my studio has its job.
Drums: AKAI S-1100 and Cubase 6.5 with its Groove Agent One
Synthbass: Waldorf Blofeld, Roland SH-101
Realbass: NI Scarbee Pre-Bass, Emu E6400 and Roland SH-101
Pads and Chords: MKS-70, Roland D-550, S-750 and Korg Wavestation. Steinberg's Padshop...in the last time very often.
Leads: Roland D-550, Waldorf Blofeld, Dune LE and MKS-70 for the 80s leads and stabs.
I don't know why, but the synth that I love most even today so much - like the first days after purchasing it - is the Roland D-50/550. It has 12-15 sorts of sounds, that no other synth can do. Maybe the D-50 is my "synth-wife"... I'm married with this synth