Margate is pretty awful, but then I grew up in Kent, and it might well have some art deco stuff. Whitstable is a bit chi-chi these days, I hear, all oyster bars and the like. Herne Bay I associate with driving tests, but it looked pretty grim even aside from that. Folkestone's probably my favourite place on the Kent coast - cliffs, faded glamour, definitely a cut above.
Yeah, Margate is pretty grim, Ramsgate and Broadstairs are marginally nicer IIRC. Deal is quite good - a little english seaside town with a stony beach, a pier with a cafe at the end and north easterlies that could cut diamond. The ideal time of year to go there is about Christmas.
Herne Bay is a non-starter apart from having a second hand record shop called B-Side the Sea Side. It never had any glamour to fade as far as I can tell. Whitstable is beyond chi-chi, but it is quite pretty and there's a pub right on the shingle called the Neptune that does really good sheps beers and excellent crab sandwiches.
The only time I've been to Folkstone was for a chess tournament there when I was about 15. That was in an old hotel, though, and pretty much chimes with your description.
Where in Kent are you from?
Other - would love to go to Romney Marsh and / or Dungeness, haven't been since I was a kid (apart from a quick trip to Rye) and I think I'd appreciate it more now.
Would also like to see more of North Essex and Suffolk - real MR James Country. Maldon is good with the statue of Brithnoth, Walton on the Naze will be familiar to Swallows and Amazons fans as the setting of Secret Water. I still need to get to Snape and Aldeburgh. There was a great K-Punk post from ages back about weird WWII era stuff on the suffolk coastline. North Norfolk is also good, but a bit of a hike for a day trip.