I lived just outside of Florence for a year in 2014, and just returned for a month, which allowed me to update and refine my "golden list" of where to eat. This is a summary of the places we tried again in 2018 and confirmed that they've still got it.
Oltrarno (the other/better side of the river)
-La Vecchia Bettola - Probably my favorite meal in a full month in Florence was at this super local spot at Piazza Tasso. Everyone knows each other, you get seated at communal tables, and the food is excellent. Quite casual. We had puntarelle alla romana, raw artichoke salad, fried artichokes and anchovies, and the penne alla Bettola which is a spicy tomato vodka sauce that was shockingly delicious. Would order everything again. Note: the coffee is not worth having, though.
-Al Tranvai- must book tiny local non-fancy place at Piazza Tasso. Great food. Call ahead to reserve. Always get: artichoke salad and fried zucchini flowers. Also, their mushroom tortallacci and pasta al'arrabiata are winners. Apparently their steak (tagliata) is amazing. House wine not our fave, but solid bottles available.
-Trattoria da Carmine- traditional local place on borgo san frediano, white tablecloth and great service and food, nice owners, lots of men eating their solo lunches there while reading the paper. Loved the puntarelle alla romana and the tartufo pasta; their tortellini in brodo was fine but less exciting. Very good desserts, made daily.
-Enoteca Pitti Gola e Cantina (across Palazzo Pitti) is great for wine and a snack.
-Osteria dell Enoteca - Pitti Gola have a new restaurant up the street towards porto romana, it's a bit fancier than other places around there but super good and great wine selection of course- prices about the same as other spots but smaller servings. Try the loro (bay leaf) digestivo. http://www.osteriadellenoteca.com/.
-Trattoria 4 Leoni is good enough, slightly tourist filled, it's Florence after all, but solid food, not cheap not expensive. Right across the square is best gelato la passera.
-Gelateria la passera, tiny corner hole in the wall. Kiwi flavor is amazing. Just go there.
-La Ditta Artiginale - only third wave coffee bar I know in Florence. Really good coffee. Apparently excellent cocktails. Near Palazzo Pitti. There is another one across the river.
-Caffe Ricchi across the square from the osteria santa spirit- old favorite but I wasn't as much of a fan this time. If you go, though, their budino de riso (sort of a rice pudding in a pastry shell) is yummy.
Not recommended (based on price)
-Omero is double the price and not quite as good as many of the places above. This ended up even more expensive than Cibro Caffe, and really Cibreo is a far more spectacular. And with an 8 euro bottle of sparkling water, it makes you a bit annoyed. It's a fun walk though, up the hill to a village beyond San Miniato Church- but still, not worth the money. If it were half the price, I'd probably go back, it was good food, but not the best. If you want to go up a hill for food, see my tips below in Settignano.
Old Oltrarno faves I didn't try in 2018 but would happily return
-Osteria Santo Spirito- seems open at all times. Huge portions, go with the half portion. Good for groups, solid and cute and casual.
-Great gelato at the santa trinita bridge in oltrarno. Have the black sesame.
-Il Santino Bevitore and Il Santino - Via Santo Spirito 64/66. http://ilsantobevitore.com Santino is a very tiny and hip little wine bar with snacks; next door is their restaurant. Both have excellent wine lists.
Duomo-side of the river
-Verrazano- Coffee/bakery/foccaccia/lunch, by the duomo this is a respite from the throngs of people. It's expensive if you sit. Try their cecina (garbonzo pizza sort of) which is in their focaccia counter in the back. Their coffee and cakes are great. Everything good, really. Sitting there costs much more than ordering and eating at the bar.
-Cibreo caffe- Expensive but delivers, it's a 40 euro prix fixe, owner is Florence's celeb chef. Don't go to the ristorante or the trattoria, caffee is the way to go. Trust me. It's really delightful, amazing service, great food… I say go.
-La Sostanza- known for bistecca alla fiorentina, artichoke omelets are AMAZING. Book in advance. It's a classic old school casual place. Their butter chicken is supposed to be famous, if you eat birds. I do find it a bit expensive, but return regularly for those omelets.
-All'antico vinaio- via de'neri 65. Famous sandwiches. Open all day, sometimes a line, delicious and cheap. You can also sit there and drink cheap wine. Sandwich shop next door to their sit-down place. http://www.allanticovinaio.com/it/all...
Just outside of Florence, walking/bus distance
**Settignano** a forever favorite (especially for hike-to-food tourism)
Just outside Florence, (20-minute bus ride) there is a little village called Settignano with a great restaurant that we love more than all others, and with magnificent walking with hills and olives and grapevines around you. Take the 10 bus from Piazza San Marco in Florence to Settignano (end of the line).
Caffe Desiderio is across from the bus stop at the end of the line (closed Monday). The owners, Michele and Francesca, are a lovely couple. He is a talkative (in the best way) food-nerd and sommelier. I'd suggest letting him pick the wine. He will bring 5 bottles to your table, describe each, and let you choose the best fit for you. The food is impeccably sourced. It was our local and favorite over the entire year there. A long leisurely lunch is great. I'd book before making the trek out there. Lunch or dinner, but keep in mind the last bus back to town leaves at 10 pm, so be careful not to miss it (its a solid 45 minute walk down the hill, which can be fun). NB: Around Settignano, there are great hikes. I walked those hills daily for a year. I'd suggest taking the bus up, hiking (walk past the Villa Gamberaia, up the hill, take a left at the fork, and just wander and you will loop back around into town). The views are great, typical Tuscany. You can walk before or after lunch.
You can also hike from Settignano to Maiano (maybe even from Florence to Maiano?) and eat on the balcony at the Fattoria de Maiano, where they also sell amazing organic olive oil from their farm. Lunch is great for the views. I remember it being quite meat-forward and solid though not spectacular food. It's really beautiful up there.
A fun hike-up-the-hill (or bus ride for the lazy/injured/overheated/tired) from Florence, or across the hills from Settignano, is Fiesole. Great views down over the city.
-Bakery called Pasticceria Alcedo- their chocolate blackberry (mora) croissants are to die for, I'd walk an hour in the rain for those if served warm. It's a stand up and eat your pastry and coffee place, no sit down.
-Slow food casual place, food is variable, some things are solid and the atmosphere is nice: Vinandro. I wouldn't make the trip up just to eat here, but if you end up there and want a meal in a sweet little place, its nice, and in summer some limited outside seating.