I asked the very same question only a few days ago at alt.satellite.tv.europe. I had just received a new receiver and was trying to save myself some time with the tuning. Anyway I've gone through 20+ satellites and these were the strongest transponders for me. I have a motorised 80cm dish with 0.6db lnb. I'm sited in Bedfordshire and use a Finepass twin tuner receiver. Obviously transponder strength will vary depending on location and I do not have every transponder programmed.

Here they are:
45e 11.495V 3254
42e 11.135V 4444
39e 12.524V 27500
36e 12.695V 3030
28.2e 10.773H 22000
26e 11.919H 27500
23.5e 11.675H 27500
19e 10.832H 22000
16e 11.554V 28800
13e 10.758V 27500
10e 11.054V 4375
7e 11.175V 27500
5e 11.766H 27500
1w 11.189V 2142
5w 12.543H 27500
8w 11.556H 27500
12.5w 11.132H 3255
15w 11.494V 17464
18w 11.661H 5632
22w 11.613H 1655
27.5w 11.665V 12223
30w 12.012V 27500