My top plugin (approximately) in the sequence of frequency of use:
1) Element 3D
2) Trapcode Particular
3) ReVision Deflicker (surprisingly often needed to smooth out flickering 3D renders from Element...)
4) Optical Flares
6) Magic Bullet Suite
7) Freeform Pro
Stardust is a recent acquisition, it may climb onto and up this list fast.
Duik is cool if you to character animation. If you do a lot of live action videos I like Digital Anarchy Beauty Box. It smooths out peoples skin really well. Digital Anarchy also has a flicker plug-in that comes in handy every now and then.
I'm assuming you mean in addition to the pre-loaded plugins... Besides Element and Particular:
- Knoll Light Factory or Optical Flares. They're basically the same, but much better than the built-in lens flares.
- GeoLayers (technically a script, not a plugin)
- Saber (free)
- VC Reflect (free)
- FX Console (free)