Otto Marchesi - I posted about it HERE. The ready to wear store is located on Viale Spartaco Lavagnini, 26r and Via di Serragli, 7, in old Firenze for made to measure and bespoke.
Liverano - One of the most recognised menswear boutiques in Florence. You can go in and meet Antonio who cuts 6 days a week, and, if you're lucky (and he's not too busy), Taka.
You will find the store on Via de Fossil, 43.
Sartoria Rossi - Like the Italian version of Ralph Lauren, but less preppy. Let's just say they love adding colour and pattern into the mix. The location is Via della Vigna Nuova, they also have boutiques in Rome and Milan.
Gutteridge - Perfect for the man on a budget. A great mix of colour and classics, Gutteridge is located in Naples vis Scotland, but you can also find their store on Via della Vigna Nouva and in most other Italian cities.
Boggi - Another big name in Italian ready to wear. Boggi has an average plus price point and these guys are truly everywhere in Italy. The stock is price point/quality perfect and can be found yet again on via della Vigna Nuova.
Simone Righi - This boutique is manned by Simone, formerly of "Tie your Tie". The store is full of sprezz and classic menswear. Very close to Vigna Nouva and just around the corner from Liverano at via de Federighi, 7.
Stefano Ricci - A worldly name in tailoring and menswear, Ricci is very expensive and only for the rich and robust connoisseur of uomo sartoria. Via Faentina, 171, it's a must see even for only their ever amazing window displays.
For most of the high-end shopping and labels head down Via de Tornabuoni or head over the Fiume Arno into old Firenze for local artisans like Stefano Bemer etc.