My favorite heirloom has to be Matt's Wild Cherry.

Not really heirloom but Black Cherry is very good and Sungold as a hybrid is delicious.

Dean

I would definitely recommend the Riesentraube. These are the most delicious cherry tomato I've ever tasted. They're addictive though and you wont be able to stop eating them!! Better than potato chips, right??

Here is a link that might be useful: Heirloom Cherry Tomato Seeds