Agreed, of the three I have some experience with - N900, N9 and Jolla - the N900 has by far the best UI. By a huuuuuge margin. It is also by far the easiest to develop for.

In terms of usability, the best UI I have ever used on any mobile device was on Palm OS. I did not care much for the frankly appalling graffiti abomination but my phone had a QWERTY keyboard and was designed in such a way that I could hold it in one hand and thumb type while walking, without looking, holding a suitcase in the other hand. The size, spacing and layout of key bumps were optimized for that purpose, unlike any other phone I used before or since, including the N900. To such details that for example, in the numeric mode, you did not need to press Shift to enter a decimal point (unlike the N900). Black text on a white background made reading the text a doddle, despite a low screen resolution (only 300x300). Yet again, unlike on N900 or Jolla. Key shortcuts, a ringer switch, contacts grouping - you name it, everything was thought through to the last detail. It may not have been "pretty" by today's standards (just plain flat graphics, no frills), but it was functional. In a way, similar to Maemo. Sailfish, on the other hand, takes the opposite extreme - aesthetics above all, at the cost of function and usability.

Why oh why do "modern" UI designers have to reinvent the wheel when they have so many excellent examples from the past to use for inspiration? I suspect in most cases it is because they are not aware of them but then it is still their fault: one should always do some research first before jumping into designing anything.