I myself, am on SMP5. I'd say the perk of SMP5 is bamfurling. That's also my favorite. What are y'all's?
I chose SMP8 (back when you had to register before joining) because it had the least amount of players and it was quiet. It's like that again.
Marriage fish. I don't see bamfurlong as a home smp perk because I can use it freely even though I'm from smp8. What about a perk that's solely for people from that server? Doesn't exist for anyone except smp8'ers. The smp8 culture is so funny.
SMP2 has Shel and... uhm... what else does SMP2 have? marks utopia as home server for tax purposes The sun never sets in Utopia, so you don't need torches!
SMP4. The best of all the SMPs. The major perk is I live there. We also have some really good builders with expansive multi-res cityscapes. There's nothing, really, that makes SMP4 stand out. It's not SMP1 ("literally #1"), nor SMP8 ("Warning: Marriage"), nor Utopia ("FLY!"). It's a nice, quiet, suburban, SMP that is just perfect for me. Still the best though.
SMP3 has a good crowd of quiet builders or, considering the frequently low TPS, a crowd of AFK farmers.
What do you mean with 'register before joining'? What is bamfurling/bamfurlong? I'm not sure what's my home smp myself.
Back then, EMC was "whitelisted" and you couldn't connect without registering on the forums first. Bamfurlong is a public villager trading farm (pumpkins to emeralds) for XP and emeralds.