SMP 1 and 9 are the most popular. You can't really argue that. Just because SMP 6 is recommended doesn't mean it's instantly the most popular and judging by online players it's still pretty low compared to SMP 1 and 9 at peak times.
SMP6 is no where NEAR the most populated server. SMP1 and SMP9 are the most populated servers. SMP6 is the least.
If you check the servers page, it shows smp1 and smp9 have the fewest open residences but smp9 has the most people on. I'm sure at different times of the day this changes and this also doesn't take derelict residences into account or people from other servers playing on another temporarily.
utopia is my favorite, mainly because everyone talks in supporter chat, just cause you cant see em talkin, doesnt mean they arent
I think it is SMP-1. I have always loved this because it is where I started a long time ago. I also like it because of the economy and the chat.
It depends what timezone you are in. Once Utopia had 15 people and smp1 had 5. That was late in the night one day. And other times. I have seen smp9 go with 3-5 people for 3 hours strait.
Smp1 and Smp9 are very popular if you are talking about players on at a time. Smp2 was like that and then it just died.