My favorite DT (19th century USA) is the Scott #2(B) double transfer in POST OFFICE. I like it when the double transfer is obvious from a mile away.
my second favorite is Scott #7, 89R2. Nienken claims its the largest, but I think the one above (Scott 2(B)), is more striking in appearance. My guess is Winston or txstamp could supply us with a wonderful pic of 89R2. It is high on my want list!
The 3c DTs above are cool, too, but I'm still a bigger fan of the the 1c and 10c
If one really wants to see an example of the triple transfer one inverted, look at lot 56 from this Siegel auction to benefit the Postal Museum from December. Lot 56. There were some other great DT's in this sale as well.