Has everyone just used plain sardines in Olive oil or the smoked Sardines?
Use good imported sardines packed in olive oil. The best are imported from Spain and Portugal. Smoked sardines don't match well with fennel. If you can find fresh, better still.
I love pasta con sarde, and order it whenever I have the opportunity. The first time I ate it, though, was on Christmas Eve in landlocked Bavaria, where fresh sardines are only a faint memory from southern shores. A friend from Sicily cooked it for us, and I am quite certain she used sardines packed in salt--they came in a large can. I have seen them in Italian delis here.