I liked Sanaa solo. Great food, and it's fun to watch the animals, although it's only 4-top tables next to the windows, so you're not RIGHT at the window as a solo diner.
The Wave was also good solo. So was Via Napoli.
I tend to go for the quieter places when I'm solo. The only place I've eaten at solo that was awkward was The Plaza, because the tables are right next to each other. But at the same time, the table next to me struck up quite the conversation, so it wasn't all bad. And I would still return, because the food is really good. But I don't eat much, so sandwiches work well for me. It's also pretty inexpensive for a table service.
One warning: solo diners seem to get the shaft in terms of tables. I've been near a door to the kitchen more often than not when solo.