The obligatory FM synth. Everyone should have one of them. There's oodles written about FM synthesis out there so the basics should be easy to find out there. We like this page a lot but beware the popups. Given that you know what FM is, the 81Z is a 4-op FM synth and, hence, rather unfashionable in this day and age but is pretty much the weapon of choice for most of our bass sounds.
Remember the bass sound on hundreds and hundreds of dance music releases from the early '90s? The slightly blippy one that cuts through a mix? That'd be SolidBass (a Yamaha DX100 preset). That is the foundation of dozens upon dozens of the bass sounds that we use. The 81Z's version of SolidBass is called "LatelyBass" and it's a damned good sound and an excellent jumping-off point.
It also does some fantastic organ sounds and miscellaneous effects.
Technically? 4-operators (oscillators for the analogue heads out there) and 8 algorithms (that's ways of sticking the operators together). There's an envelope per operator, an LFO and all sorts of other goodies. Do remember, too, that most FM synths only offer sine waves as operators. The 81Z offers 8 spiffy waveforms to use as starting points. This means that you don't have to waste operators getting basic (but non-sinusoidal) waveforms together.