A well-dressed man could politely be described as "dapper", "debonair", "dashing" or in a more informal manner as "snazzy". A well-dressed woman might politely be described as "classy", "sophisticated", "stylish" or if you are really impressed, you could say she looks "stunning". Hope this helps

Wow, you look amazing! You look breathtaking! She really looks fab tonight! He's lookin' good in that shirt.

thx

Fancy, chic, smart, elegant, extravagant, magnificent These were the only ones I could think of :)