My top 3, in order from merely great to truly sublime were:
3. toasted lobster roll from the Clambake in Ipswich: at $12.99 (sans french fries) this was an incredible bargain: perfectly buttered/toasted roll overflowing with a no-frills lobster salad; heavy on the meat, light on the mayo; added a bit of hot sauce for extra punch
2. Valrhona bittersweet chocolate ice cream cone at Christina's: incredible flavor and texture; best ice cream experience all summer
1. tofu banh mi from Mei Sum bakery (thanks to Chowhound recs for this one): this was my first banh mi ever, so I've no point of comparison, but it was kind of magical the way the thing just worked so well as a whole, with many distinct flavors and textures all perfectly balanced: fresh, hot, sweet, crispy, herby, and only $2.75! I've found a new lunchtime fave...