PokerStars is the biggest "stand alone" site out there. No other sites uses their network.
It's hard to give advice as to which one to choose and it's hard to say which is "most similar" to PokerStars in terms of software design because these issues are so subjective.
From personal experience PokerStars, for me, does have excellent software (being able to change table size to exactly how you want it has got to be a good thing) and it has nice skins to download too.
But Nevertheless, for a "sleek and classy" online poker experience I would have no hesitation in recommending Poker Heaven. It has a silky-smooth, slick colour scheme (delicate biege, deep red, black), the interface is very easy to navigate and user-friendly, the chips move around nicely and it's just satisfying to operate overall. Plus, it has a very good TV channel! It also looks superb in full screen mode!
It may not be the most well-known site, but for a classy online experience it's top of my list.
(By the way, it's part of the BOSS network so any site that works under this name is probably good.)