Best place for filters?

Hi, everyone!
I've been using an MK-67 (with 2" Intes diagonal, 1.25" and 2"
eyepieces and 2" Barlow) for a few years now, and I'm considering
getting some filters (UHC, colour), finally.

I'm wondering where in the optical train would be the best spot
to put them, to keep their imperfections from deteriorating
sharpness and contrast as far as possible.

I've got filter threads:

- on the diagonal (facing the OTA),
- on the back of the 2" - 1.25" adapter,
- on the Barlow,
- and obviously on the eyepieces.

Apart from convenience aspects (like the ability to change eyepieces
while keeping filter in place if it's mounted in the diagonal),
what would you suggest? Is there any noticeable difference at all?
Should filters be kept away from the focal plane? Kept close to
the focal plane?

Cheers
Steffen.