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The ideal eyepieces for a given person vary depending on your focal ratio of your scope(s) and not so much your focal length of your scope(s) as has been mentioned (I think). I think it is sometimes best to think about your eyepiece collection in terms of exit pupil and that is a sole function of your scope(s) f ratio(s) and the eyepiece focal length.
Exit pupil is the eyepiece focal length divided by your telescope's f ration. So if you have a f/7 scope then a 15mm eyepiece has a 15/7 or about a 2.1 mm exit pupil. For an f/3.6 fast scope the same 15mm eyepiece has a 4mm exit pupil. This makes a big difference in terms of your eye's ability to use that light energy.
Try to be sure to have something in the exit pupil ranges of 4, 6, 8mm exit pupil range and if you do really deep stuff with larger scopes even have a favorite eyepiece in 2mm exit pupil range for deep sky if you are good at averted vision like most deep sky observers tend to be.
A dark adapted eye of a an observer of 40 years or younger will be around 6mm and as you get older it gets usually smaller like 5mm by age 50 (these are all just averages, your mileage may vary).
When you get to an exit pupil of less than half the diameter than your own dark adapted eye the view becomes darker and darker as you are able to utilize so much less than your full eye's dark adapted pupil.
When the exit pupil is greater than your eye's opening this is where you search around in the field by moving your head and don't see the whole field. This can be it's own fun but I'm just explaining the mechanics of it here.
So the above reason is why it is thought that much less than 2mm exit pupil looses too much light to be of value BUT if you have massive aperture that of course compensates.
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