I personally highly recommend the Lee filter system. I spent a lot of time years ago looking into filter systems. It ultimately came down to Cookin Z-Pro and Lee. In the end, I chose Lee, as at the time they seemed to have the most flexible system (tandem adapters, hoods, configurable stacking options for each holder, a flexible CPL option, etc.) They still have most of the same advantages today.
The really nice thing about the Lee system is it is adaptable to lenses with up to 100mm filter thread diameters with a special adapter, and up to 82mm with a standard adapter ring (such some of Canon's newer L-series ultra-wides). It can also be adapted to lenses with very small filter thread diameters. The one single system can be used with pretty much every lens except those that use a rear filter slot, like Canon's supertele L-series lenses.
I've used the Lee holder, in different configurations, to hold anywhere from one to seven filters. One to four with a single holder, and up to three in one and four in the second for two holders in tandem. You can change out how many filters are stackable by guying different slide guide filter screws, and changing out how many slide guides are on the holder. For really ultra-wide lenses like 16mm (or 10mm on APS-C), this flexibility is useful, as dropping to two slide guides helps reduce the vignetting that occurs, while at the same time offering more stackability with longer lenses, the tandem option (which can be useful when you have a non-even horizon, allowing for different GND angles), and the ability to use Lee's very nice (but also very expensive) CPL filter in tandem with a 4x6" holder.
One of the things that I've found useful with Lee's system is the ability to add hoods. Sometimes this is very necessary, as when you start stacking filters, especially non-coated resins, the chances for flare greatly increase. The hoods allow for a lot of flexibility as they are bellows-like, and can be adjusted for different lenses. They don't generally work with really ultra wide lenses, but they work with most.
The Lee system is compatible with any 4x6" filters as well as 4x4" square filters. The former are used for GNDs, reverse GNDs, and a couple other unique options, where as the latter are used for solid ND filters, polarizers, gel holders, etc. You can use Lee, Singh Ray, Hitech, and a number of other brand filters in the Lee system. For DSLR-size lenses, this filter size is pretty ideal. They are a bit large for 67mm filter thread lenses, but are just about perfect for 77mm filter thread lenses.
You can get a starter kit for Lee Filters:
http://www.bhphotovideo.com/c/product/491474-REG/LEE_Filters_SET_RESIN_DIGI_Digital_SLR_Starter_Kit.html
When I purchased this, it was on sale, and a LOT cheaper. I don't even think I spent $200 on it. So, a price of $320 is well up there, seems a bit too much to me, but, it's the same kit I started with. I've added quite a number of filters over the years, namely the four primary GNDs in both soft and hard grad: .3, .6, .9, 1.2. I recently broke my 2-stop Pro-Glass ND, and I have a couple of other Hitech ND filters (including a 10-stop, which turned out to be an IR 10-stop that doesn't work with a standard color bayer sensor), none of which I really like...Hitech filters just aren't the same quality as Lee or Singh Ray.)
You want GND filters for landscapes that have a good horizon with a strong contrast differential (high dynamic range scenes), so you can bring the dynamic range within the range of the sensor. This works great when you have an even horizon. When you have an uneven horizon, soft grads are more useful than hard grads, especially the ability to blend a couple of them together. A reverse grad is useful for coastlines. Solid ND filters are very useful for allowing greatly reduced shutter speeds for longer water exposures. A 10-stop filter is useful for coastlines where you want to fog out the motion of the waves super-blur clouds, and create other effects with the in-motion parts of your scene. (Just, beware of Hitech 10-stop filters, it can be difficult to figure out which one is which, if you get the IR (infrared) one, it just doesn't work with normal cameras.)
