Season 1 through 3 (Episodes 1 - 63) are all closely related to the manga, and is essentially a crucial point in the anime as it establishes and introduces most of the reoccurring cast and themes in the show.
Season 4 and 5 are filler arcs about Bount (essentially spiritual vampires) and are hit-and-miss. If you want watch it you can, but there nothing significant that carries on towards the main story line.
Season 6 through 8(Episodes 110 - 146, 152 - 167) return to the main story line told in the manga. Episodes 147 to 151 are filler episodes that are not bad, but do not convey anything to the main story.
Season 9 (Episodes 168 - 189) is another filler arc the completely breaks away from the main story to tell another story (so essentially, it's completely non-canon) and it is interesting. Again, if you just want to see the main story, you can skip this, but this arc actually is on of the more interesting arcs in the series.
Season 10 through 12 (Episodes 190 - 203, 206 - 226) continue the main story (with Season 11 deals with events that occurred before the main story). Episode 227 is another "origin" episode that tells how most of the main cast met and some events that are key to the start of the series. Episode 228 is the obligatory beach party episode and episode 229 is just another filler.
Season 13 (Episodes 230 - 265) is another filler arc that deals with the sword spirits of many of the main cast. This is another interesting arc, if only to see the unique spirits that all the swords contained and personified. Again, skip it if you like, noting will be lost here.
Season 14 (Episodes 266 - 310) continues the main story. Episodes 311 - 316 are all filler episodes, so the real ending to this season is Episode 310.
Season 15 (not sure how many episodes) is yet another filler arc (kind of strange considering what happens in episode 310).
So there you have it. I hope this will help you to guide through all the unnecessary filler and just enjoy the main story. Note that many of these events are connected while others are not, so missing a few episodes could get you lost. Be sure to see the first 3 seasons because you will meet a lot of the huge cast again.