For me, I can get away with using these as the essentials:
1. Preamp / Amp Sim (VT Bass) - makes my Markbass amp sound like an old school Ampeg, and is great for sending an amp-like tone to the PA, or putting into the FX return to bypass the preamp of crap amps
2. Overdrive (modded Bad Monkey) - essential for tunes that need a bit more grit, or for boosting choruses/solos etc
3. Fuzz (Supercollider Beast) - for when something more than light grit is required, and for creating noise loops with the oscillators
4. Delay (SMMH) - Used for a bit of delay/reverb in ambient parts, and for creating noise/drone loops with the Supercollider. It can also be used for chorus/flanger sounds.
I also have a Wolly Mammoth clone, Tonefactor 442 Filter, Boss OC-2 Octave and CE2-B Bass Chorus which are great for using with the above to make synth sounds for shits and giggles.