[wavemachine]

Depends on what scenes you are shooting really, OldBoy is right you can get cracking colours at Autumn but with your 7D shoot in RAW as this gives loads of scope to tweak white balance and apply other boosts to really bring out those rich colours and because of the extra bit depth you can boost things a bit further without introducing extra noise.

If you are shooting scenes with blue skies and don't want to mess around with the image too much then filters to consider are a set of ND Grads (hard and soft ND2, 4 and 8) and a circular polarizer personally I have a Cokin system and have not suffered the colour cast others have had however the best filters are with out a doubt Lee although these are expensive and there is quite a waiting list. Another system is hi-tech, I would look at the 100mm square filters these are cheaper than Lee and are pretty good.

The other option is to buy screw in filters, you can buy the size for your biggest lens and use step down rings to avoid buying the same filter over and over again but ND grads can be tricky to line up with screw in filters so I would be inclined to look at the square filters.

Filters are expensive and do add up in cost very quickly so probably best to decide on a system and build it up over time.

If you do really want to use filters then an ND4 soft edge would be a good starting point and maybe a polariser, but I think as old boy said with the correct light and the rich autumn colours you will be rewarded with great shots.