What's available? Fresh from your garden would be best, farmer's market a second best choice, fruit and veg stands #3, and then whatever you can find at the grocery store, preferably locally sourced. The further they have to travel, the more likely they are to be picked while still somewhat green and 'ripened', which adds color but IMHO not much flavor.
I've been buying the 'tomatoes on the vine' ones at the store since last season, those seem to be the most likely to taste like a tomato. Smaller store-bought tomatoes tend to be better than bigger ones, too.
