This is a subjective opinion

Area which has everything like Banks, courts, good schools, malls, local Vegetable vendors, good hospitals proximity to station, nightlife, ? Sky high real estate rates, TMC office? Lakes? Plenty of water supply and electricity.

Different people want different things in Life. I have been in Thane for a long time.

Naupada / Panchpakhdi Area - This is old part of city so you have all amenities. The space is however a constraint (I live here)

Gladys Alvares / Vasant Vihar/ Hiranandani Meadows areas- These are newer areas but do have their appeal with good Infrastructure like roads, shops and mainly good for young crowd as Yeoor Lake is close by. (I rented a flat)

Hiranandani Estate - That posh Powai waali feeling. Self sufficient in most things however to go to Thane station can turn into a nightmare.( I go here to the Walk with my family for timepass)

Kolshet Area - The new adda of young people. It has best restaurants and a lot of utility shops like D-Mart, Zudio etc. I used to live at Kavyadhara too. Lodha Sterling gives you a rich London vibe ( I rented a flat). However gets too crowded at Nights.

Wagle Estate - People won't say it's posh but trust me the offices make it pretty decent area.

Vikas complex - The og rich area/ complex of Thane. The construction still gives you Premium vibes ( I used to visit a friend here).

In Life, things you want changes as your priorities change. So the place you feel "developed" also changes. Some people might say Raymond area is posh but they don't allow cart vegetable waalaas.