Zone 6 is an excellent zone for growing tomatoes, overall. Your biggest question is your soil. If you amend your soil properly, you can have excellent heirloom tomatoes.
Pick your varieties based on your taste preference. For a true tomato fan, I always recommend trying any of the following... Black Krim, Black from Tula, Purple Calabash, Omar's Lebanese and Persimmon. There are so, so many out there to try!
Best wishes and Happy New Year!
DD

My absolute favorite in the Chicago suburban area is Pruden's Purple. We think the flavor is great and it produces well and reasonably early here. Check it out...

Jean

My favorite heirloom is Amish Paste. It's not just a paste tomato, they're very good on sandwiches, etc. Sungold is the BEST cherry tomato ever! People who don't even like tomatoes like Sungold!

I read an article about taste-testing paste tomatoes. Both Amish Paste and San Marzano were tops. I've grown both and they are far superior to Roma.

Rhoda

Rose de Berne has great taste.

Tappy's Heritage yeilded really well for me and tastes good too.Also try Japanese black trifelle, black cherry,I really like white tomesol.Was really popular at the 2004 Heirloom garden show.