If you were to use only one filter, Celestron's Skylight Filter/Dust Seal would be your most versatile choice. This general purpose, Wratten #1A filter improves color saturation and balance, increases visual and photographic contrast, helps to decrease glare in lunar and planetary observing, and penetrates atmospheric haze.
It's also an inexpensive way to protect your investment in quality optics. This filter threads onto the rear cell of your Celestron C5, Nexstar 5, 5i, C8, Nexstar 8, 8i, GPS 8, C9 1/4, C11, GPS 11 or C14 telescope to serve as an effective dust seal.
By leaving the Skylight Filter/Dust Seal on when removing or attaching other visual accessories, you prevent dust and other atmospheric contaminants from entering the rear of your telescope. Needless to say, it's much easier to clean a dusty filter than a dusty primary mirror, secondary mirror and corrector plate.
The Skylight Filter/Dust Seal must be threaded onto the rear cell of the instrument before any other accessory is installed. All additional accessories for example, 2" diagonal, reducer corrector f/6.3, and visual back, mount directly on the outer thread of the sky/dust filter.