I'm with Marty - NTSC is the only filter you actually need. If you can't handle what NES games really looked like, don't play 'em .
This isn't a matter of taste - the NTSC filter is almost exactly what the games truly looked like in the 80s when we were playing them on real h/w. The other filters all scramble the image in an attempt to make games look good to people who weren't born yet when they were released.
Those comments are silly, because for one, not everyone who grew up playing a real NES did so via an NTSC TV. There's also PAL.
As well, I was born in 1977, and I grew up in the U.S. playing real NESes. Yet I still don't use Blargg's NTSC filter, for a number of reasons. For one, on my computer (2.8 GHz P4 with 512 MB RAM running Windows XP), Nestopia with Blargg's NTSC filter enabled runs ultra-slow. Another reason is because NTSC TVs have crappy fidelity.
Trying to simulate NTSC TVs is a bit like trying to simulate old audio recordings being played through the old horn phonographs when we have the master recordings available and modern high fidelity audio equipment to play them back on. In this case, we have the bit-identical ROMs available and much more accurate RGB computer monitors.
It might be replied that the game designers were targetting the NTSC TVs (or the similar Japanese TV standard), and hence the colors they chose in their games were picked using that standard. That may be the case, but it's unlikely that's how most of them actually desired their games to look: rather, they were simply making do as best they could with a low fidelity playback standard.
To use a precisely corresponding analogy, it would be the same as trying to simulate the 1980s U.S. TV shows (e.g., The A-Team, MacGyver, etc.) being played through NTSC TVs. Or rather, saying that if you don't play back the DVDs of those shows simulating 1980s NTSC TVs that you're not getting the colors right, i.e., according to how the directors of the shows intended them to look.
So nowadays we have the ability to make these games look much better than they did when we were originally playing them.
I personally don't like using any filtering method other than nearest neighbor interpolation on any of the emulators I use. It produces more noticeable pixelation, but it keeps the picture as sharp as can be. This looks by far the clearest (i.e., no blurriness at all). Plus, it's the easiest way to resize an image, and so runs the fastest.
I wouldn't use nearest neighbor interpolation for photographs, because it doesn't look natural. But these old video games already look unnatural, so nothing is lost using nearest neighbor interpolation on them.