Good question, tco, and the answer may of course depend on individual circumstances (e.g. horrendous extenuating circumstances for the past couple of years, so there might be a need to go back to years 2-3 to show what performance was like prior to these events).
Broadly speaking, though, I would focus on year 5 and as much of year 6 as is available.
Panels do like to see a complete year 5 report (although it will be of limited value if it's one of those that makes little mention of achievement). Apart from this, as far as individual subjects are concerned, try and keep the focus on the most academic subjects, especially English, maths and possibly science.
Good CAT (or other reasoning test) scores, and good reading ages, no matter what year, are always very useful as individual results of this kind are 'snapshots' and may not be wholly reliable. It helps here to see a consistent pattern.
Optional SATs results from just year 5/6 would usually suffice, if they're good enough.
No need to write much in the appeal submission about all this - the attached academic evidence should speak for itself.
There's a specimen appeal submission in the Q&As that might be of interest to anyone who's not yet seen it:http://www.elevenplusexams.co.uk/11plus ... rs.php#e11