There are a couple posts here already on what filters are available and why you would use them, having been around the block a couple times on filters used for outdoor photography I'll try for an answer from a different angle: what I actually wind up using.

Polarizing filters: Loved them at first, never use them now because they give an unnatural look to wide angle photographs that include skys (giant dark blue gradient in skys) and cause problems w/stitched photographs. Still nice for tighter focal lengths or shots w/out sky, especially of foliage but overall I just never carry one.

Split ND filters: Great idea in theory, kind of a pita to use in practice, especially if your exposure differential isn't a straight line (flat horizon). I'm sure these were more useful back w/film where you had to get it in one shot, but now I'd rather take 2-3 shoots and create the gradient in post, not necessarily HDR but thats an option too. Plus cheap split ND filters are resin or plastic (who wants plastic in front of a $1000 lens?) and just one SND filter doesn't cut it, you usually need 2 or 3, so to get 2 or 3 nice glass ones plus a holder... $$$.

UV/Clear filters: never use these anymore because they're so prone to flare. The only time I take one is if I'm shooting near something that might spray the lens (ocean).

ND Filter: This is the only filter I actually carry anymore because if you're already at ISO100 and stopped down to f16 or f22 and you still can't get a long enough exposure, this is the only way to do it. Or if you want to shoot at a wide aperture in bright daylight this is the only way to reduce the amount of light. I picked up a 10-stop ND filter and its awesome. Another option is to get a Polarizing filter and just turn it till its polarization is 'off' and this'll give you 1.5-2 stops of ND.

My two cents.