Hi Danial,
I gave a presentation at IISAC a few years ago about Visual astronomy and the use of filters. A copy of that presentation in PDF format can be
downloaded from this thread. This will give you a better understanding on what filters are suited to what purpose. The critical thing to remember is that no filter will make the target brighter. They make the detail in the target more visible against the background by removing all the unwanted wavelengths of light.
For the person owning only one filter I would recommend the DGM Optics NPB filter. This is a Narrowband (UHC) filter with a very tight bandpass. It is the best of all the narrowband filters that I have used which is just about all of them. It works well in both small and large scopes and works well with a wide range of exit pupils. I have used it successfully with exit pupils from .5mm to 6mm. The DGM NPB filter has a high bandpass in both OIII lines at 496nm and 501nm and importantly it has a high bandpass in the H-Beta wavelength at 486nm. This makes it a good option in the absence of a dedicated H-Beta filter on some selected targets with high H-Beta emissions. Some of the other Narrowband (UHC) filters do not have such a high bandpass in H-Beta.The Orion Ultrablock is another good choice, as is the Lumicon UHC. I rate both of these as being slightly behind the DGM NPB.
You can order the DGM NPB filter direct from Dan McShane the owner of DGM Optics. These filters are made for Dan McShane by Omega Optics in the USA.
Omega Optics sometimes have "blems" that get sold very cheap on ebay as Omega Optics NPB filters and there is nothing wrong with them at all, but there are none listed at present.
Cheers,
John B