For me, it has to be Leuchtturn1917's 120g/m³. Also like Rhodia, though it's a bit thin for my preference. Happy to hear you guys' suggestions for a slightly heavier (let's say 100g/m³ and above) paper, either loose leaf or in a notebook.
I love Clairefontaine paper a lot (Rhodia and also Oxford): it's cheap enough and works great with my fountain pens (F/EF). Tomoe River is an amazing paper but it's always way too expensive for any... regular and intensive use (even more so compared to Clairefontaine which is cheap enough for kids to use it in school), at least here in France.
I like Leuchtturm too, but it's also too expensive to my taste.