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It depends on where you've planted it and how much space you have. If you want it small, M9 is the classic choice: it doesn't grow much, starts producing quickly, and the apples are good. If you have a big garden instead and want a more vigorous tree that lasts for years, MM106 is better - it's more robust and handles problems better. M26 is in the middle if you're looking for a compromise.
Also consider the soil: if it's wet or tends to stay damp, MM111 is the best choice because it tolerates water. If the soil is poor or dry, the simpler rootstock performs better. One more thing - M9 might need stakes because it's not super strong, MM106 doesn't.
Talk to a local nursery if you can. They know well what works in your area and what's available. It's not a super critical decision anyway - most rootstocks work decently if you take care of it properly.
The M9 is convenient, but it has a flaw: if the soil isn't perfect, it creates anchoring problems and the tree becomes unstable in the wind, especially in the first years. What nobody mentions is that you should check soil acidity before choosing: the MM106 and M25 tolerate less-than-ideal soils better compared to the M9, which is pickier. If instead your orchard is exposed or windy and you don't want to be constantly dealing with stakes, the M26 is a good compromise between vigor and early productivity.
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