Unfortunately, the bookkeeping of my collection is not as tidy as it should be. I have all scales mixed in the same spreadsheet, and I keep discovering missing entries, as well as sold models that are still there. Nonetheless, I think it is still accurate enough to give a general picture.
1/400:
1) 747 (about 70)
2) 727 (about 50)
3) Much to my surprise it is the 737 family... (about 30)
4) 767 (just below 30)
5) DC-10 (about 25)
Below that would be the DC-9 (including the MD-80), then a tie between A340s and 707s, then a tie between the A300 and the L-1011, then 757s, A320 family, and then I believe everything is in the single digits.
My current 1/400 collection sits at around exactly 400 pieces. And it will probably stay like that for a while, as I am starting to have a high turnover ratio.