It doesn't really matter since the force is almost always "down" relative to the print. I almost always use the old "rectilinear grid" infill because it's fast and nearly as strong as others in the "down" direction.
If you need more strength, just increase infill%. I use 70% for such parts, which is nearly as good as 90% but, again, faster.
90% is IMHO the largest infill you should ever use, because it allows for flow calibration errors without your part overfilling with plastic.
The only thing I can think of is that your extrusion rate isn't calibrated. Or your slicer is broken, which is unlikely. You *should* be able to print a 10mm cube at 100% infill and have the top surface come out nearly perfect.
You may also need more "top solid" layers to hide whatever infill does.
I've got three. Always on 100% after the first layer.
I prefer 0.2mm for default prints.
I prefer 0.5mm for default prints.
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