The one that lets you obtain results you want which are not possible with gear you already have.

Your current purpose, improving skills, does not require any additional equipment at all, just like improving driving skills would not imply buying a fleet of different cars. Practicing and learning about the craft would be much more effective.

No idea where to begin? Look around for photos you would be proud to have taken yourself. Analyze what makes you like them and try to replicate those traits with the camera and lenses you have. Research what others have written about shooting the kind of subject, or in the kind of environment you're targeting to get ideas for improving your results. Pairing up with someone else interested in similar kind of photography will probably provide positive influence for both of you.

So first, try to identify what kind of photos you are after. Then make your attempts and analyze results to be sure that it really is your equipment holding you back. When you get this far, you'll already have much better understanding what kind of equipment you need to overcome the obstacle.

Sure, there are some genres in photography that will strongly suggest some special equipment. Sometimes it's lenses, sometimes something else, often a combination. Having decent equipment for all niches would be very, very costly. So picking a direction first is essential.