I've been looking for lens filters lately. I don't really want to order one online, but there is no camera shop hear in town. There is one about an hours drive from here and I am heading down that way in a week or so. They deal with Corkin Brand filters. Until I looked at there website I thought there was only on kind of filter. They have the screw in type filter and they have this kind of slide in filter system. Sorry this is all new to me. The Slide in system seem more versatile. but the screw it type "I think" would be a little more durable. Here is the link to the corkin site. What do you think?
http://cokin.com/

 

Here's another name to look into . . . B+W.
What kind of camera are you using?
(Hereafter my opinions refer to SLR/DSLR bodies.)

I've used the slide-in type you're talking about. (gradients)
They're somewhat awkward.

My personal preference is the threaded kind.
Invest in a UV (skylight) filter for each lens you own.

Thereafter, a circular polarizer is probably every photog's stalwart tool. For any wide-angle lens, I'd suggest you get a thin one. Either way, make sure it's coated.

 

Thanks, I have a Rebel Ti a 20-90mm lens and a 80-200mm lens.