I went with Katie Boulter. She and Katie Swan are the only ones I feel confident will make the top 100. That's not to say the others can't or won't make the top 100, but for me they have more to prove.

In truth, I'm not sure any of them will.
Hopefully, all of them will.

Ms. Boulter has a chance at the start of next year, if she plays tournaments at the level to which she has now entitled herself through her consistent fine performance sustained throughout this year.
WTA qualifying, GS qualifying, top tier ITF ($80K+).
If she excels there, she would naturally get very close before any of the others have a chance to have much of a say about it. Can she clear that hurdle?
It's the very steep climb from ~150 > 100 that tempers my optimism. It stalls out many very fine players, because despite seeming to perform well at ITF's you effectively tread water, and need two or three really big performances at $100K or WTA to break through the barrier.
As it was with Naomi and Johanna, it required a sustained period and repeated trips to the final stages of bigger events than any of these players have thus far shown that they are capable of. Or, one amaazing week at a WTA - qualify and reach QF, or similar.

I'd like to think that they'd step up, some or all, in time; given repeated exposure to those occasions; but I'm not entirely convinced yet.
Just have to get there and try it out!

I put a couple of quid on Katie B and Freya about three years ago - might be completely wrong but, out of that group, they were my pick then and they still are.

(Mind you, if you are going to have Emily App in that list, you should have Jodie too - she's in my KB/Freya group. Equally, if Katy D is there, Harriet should be too. Just on the basis that both have almost identical profiles to the other. Also, Fran Jones. And Maia?).

I would like to add Maia Lumsden to that list. I think she has the talent and the mentality to get there.
I think both Katie Boulter and Katie Swan are capable, but it may take some time.

On her current performances trajectory, Fran Jones would definitely seem to have the potential for reaching the top 100, but with her ranking next week being about 678 she is still probably around 2 years off, if she continues to progress as she has been doing.

In terms of the next person to reach there, Katie B certainly has the inside track at present but considering that Monica Niculescu, the current WR100 has 639 points and Katie at 199 has 285 points means that Katie has to more than double her current points haul to get there. I guess we might have a better idea after the Australian Open as to her potential and possible time period for doing so, if she gains any real points there (although if she doesn't, it doesn't signify that she won't get there, just that it will take longer).

Gabi Taylor I note has not been mentioned here, I assume because of her recent not very well explained disappearance. Up until that point she might have been my favourite to get there first, but now its been 14 weeks since she played. I note her instagram account shows her hitting forehands at Sant Cugat in Spain a few days ago, so maybe she will be back again in the new year. Of Katie B, Katie S, and Gabi, if Gabi is to play, I would still probably go for Gabi first, and then Katie Swan second. However I'd be very happy for Katie B to prove me wrong and demonstrate that she does have yet another level (at least) to her game.

It is an interesting debate, Katie Swan definitely has the junior pedigree to step up indeed we tend to forget she is still a junior. I can remember watching her play as a wild card in Wimbledon qualifiers in 2015 and her beating a top 150 opponent Kristina Kucova in straight sets relatively easily. She has the game and I am confident she will make top 100, she strikes me as a very balanced individual and unsurprisingly given the power she puts through her body on slim frame has picked up a number of ?muscular injuries, touring as a 17 year old turning 18 I also think is a shock to the system fully acclimatised I think we will see significant progress next year. So Katie Swan was my choice and in the longer term I feel will spend most of her career in the top 50.

I missed seeing Katie Boulter develop through juniors and she got injured and virtually missed the whole of 2015, only now are we seeing her potential, I think if you watch a player develop you get a better idea of where you think they will end up. I see Katie B as being stronger mentally but I feel she is closer to her optimal potential than Katie S. I am pretty confident she will make top 100 indeed she may do it earlier (which I suppose is the debate) but feel she will be in and around the level of the Nicola Gibbs. Only time will tell but both have great potential.

Fran Jones is a warrior if her hands hold out she will be right up there.

Gabby Taylor I also feel is driven and mentally very strong from a tennis perspective and I wouldnt be at all suprised to see her make top 100 in time. Freya has so much power she just needs to stay fit be consistent and understand how to play do her undoubted strengths again she is young and there is still time to grow from a game craft perspective.

Holly and Emma early days but the potential to get to a level in juniors where you would be confident of high end WTA careers

I'm not sure any of these will (please be totally wrong). Freya (with Gabi) has the biggest potential I reckon