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I would have to say none are essential, although it is surprising the difference a UHC makes when observing nebulae in the 'burbs. The contrast between the sky and target is boosted quite a lot. I tried putting the UHC on the scope at Snake Valley, and I believe it made the image worse. I guess if the light pollution in the sky is a limiting factor, it will help. If the sky is basically pretty dark then the filter will just reduce the amount of light you see...the contrast is as good as it is going to get.
I have a polarizer for the Moon when it gets bright, but when it is near full I have no real interest in looking at it anyway!
If you "had" to buy some filters, I would get an UHC and an OIII. I very rarely use my colour filters.
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