I know you're tired of the same old question, but...
Here's the deal. I am a loyal Chowhound from Austin Tx. I'll be in your great city for the first time next friday and, as usual, I need the usual advice. To preface the question let me say this, if you come to Austin and ask me where to eat dinner Friday night that will reflect all that is good about the Austin scene I wouldn't necessarily send you to the most expensive or the most outrageous, I'd send you to the most Austin. That might be to the Broken Spoke for a chicken fried steak, mashed potatoes, cream gravy and biscuits, and, of course, a few spins around the dance floor to a live Country Western band; that might be to Dart Bowl for a bowl of their etherial enchiladas, so good and gooey and cheesey and fun that you just might have to walk out of the restaurant to the front desk, rent some shoes and bowl a line or two; or it might be to Dirty's on the drag for a double bacon cheese burger dripping with greasy goodness, and a side of rings washed down with a cold Pearl beer or two; or maybe even out MoPac north to Sunflower, the best Vietnamese restaurant this side of Hanoi, try the Crepe, try the Basil Shrimp, try not to walk out smiling! See, I don't really care if it costs a lot or a little, I'd say go where the heart of the City is. With that in mind, My wife and I will be staying at the Radisson Plaza-Warwick. We are on our own just Friday night (my daughter will be with us age 26). So tell me, where to eat?
Some aspects of Philly food. There are lots more in each category but some of my picks off the cuff:
Creative fine dining: Zahav - a world class restaurant IMO
Casual Chef-run BYOB: Kanella, Bibou, Modo Mio, or Fond
Italian BYOB: lots of options in this category but I've been digging on Hosteria da Elio lately. Modo Mio is Italian too but not the same genre to me--menu is a little more refined.
Gastropub: Royal Tavern or South Philly Tap Room. Many may say Pub & Kitchen near your hotel, but I think it's too pretentious to have "soul," plus I recently had my latest bad meal of several there.
Sandwiches: DiNics, Paesano's, John's Roast Pork, Nick's Roast Beef... could go on forever.
Little Fish should definitely be on your list. BYOB. Agree with Modo Mio (and that it is more refined than Hostaria). However, don't confuse refined with expensive. Modo Mio is probably the best deal in town. Get the turista menu. Both these places are a car or cab ride from your hotel. Little Fish is in Bella Vista (south of you) and Modo Mio is on the border between Northern Liberties and Fishtown (north of you). Closer to your hotel, I like Meme.
3rd Reading Terminal Market, but that's definitely not a Friday night dinner place. Heart of the city? That's tough to say. There are so many options. I might go to Standard Tap in Northern Liberties. Or perhaps I'd go to Good Dog for their burger, then walk around the corner to the Oyster House for a plate of oysters, then walk directly upstairs to Nodding Head for some brewed-on-site craft beers. Actually, that is exactly what I would do. Philly does BYOB's and gastropubs really well. I should say there are probably better individual places that you could go, but those 3 are close and would give you a good, solid feel of what Philly has to offer. Substitute Tria for Nodding Head if you wanted wine instead of beer.
Of course, you could also do the Pat's and Geno's thing. Without getting into an endless cheesesteak debate, there are still tons of locals who go there and you can get a decent cheesesteak. And it might be the best "heart of the city" experience (outside RTM) that you can get. Either there or Tony Luke's.