I've been brewing all-grain batches for some time. With my water source, I'm generally getting good results across multiple styles. To date, I haven't done much with water chemistry. Which styles of beer will customizing my water chemistry make the most (positive) difference ?
The answer to your question is very much dependent on the water chemistry of the water you are using now. If you have alkaline water you will find that you'll have the most success with beers using roasted malts because the acidity in the malt will bring down the mash pH to the correct level (5.2-5.4). Lighter beers will be harder to brew due to your mash pH being too high.
If you have soft water you'll be able to brew most styles successfully if you have adequate calcium.
If you haven't already, the first step is to order the W-6 test from Ward Labs to determine the properties of your brewing water. If you have the results already then I can amend this answer to address your water more specifically.
There's a lot of information out there about matching color to residual alkalinity, but you can disregard that because it oversimplifies water chemistry to the point that it is not worth paying attention to.
First ask yourself of all your beers which ones are your very very best. And be honest with yourself. Using known award winning recipes make something light (pilsner or kolsch), something amberish (Red Ales, Pale ales) and something dark (porter or stout). Submit these beers to a couple competitions. See which ones do the best.
While doing that, get your water source tested from Warb Labs. Then figure out what conventional wisdom about water chemistry says about your beer style options. Refer to "How to Brew" in this regard.
You may be fortunate enough to live in an area where the water is somewhere in the middle and you don't need to do much to make great beer. That is sort of my boat. But I am curious to see if I make one of my best beers and try to make it with "appropriate" water profiles to see if it really makes a difference.
Tough tasks where results don't come overnight. But if you are serious about making really great beer...maybe these are the answers or the road to that success.