What is the axial tilt of a planet measured relative to? I am very much a beginner on the astronomy front but I understand about planets having different axial tilts, hence why Venus turns the opposite direction from the Earth and Uranus turns sideways.

However, I am confused as to what the axial tilt bearing is measured from. For example, if all planets were in a perfect line from the sun, would they spin perfect in alignment on their own various axial tilts? Would some still be 'off-set' by a few degrees to one side or the other?

Do we actually know please?
If you look at the Astronomical Almanac for the year 2011 as an example, in the table at the top of page E3, you find two measures of axial tilt. The third column is the declination of the planet's north pole for the mean equinox and equator of date 2011 January 0, 0 hours terrestrial time. So this is measured with respect to the Earth's orbit. The last column is the inclination of the planet's equator to the planet's orbit.

If you look at the right ascension column in the same table, you see that on that particular date the right ascensions are all different.