It's impossible for me to pick one. None of my top 5 are on the server though (Outside of events)
But my favorite Non hostile SCP is SCP-131, They're just cute little guys.
Favorite hostile is SCP-610, It's not impossible to contain, and isn't "Overpowered"
And the one that freaks me out is SCP-3199, It's just such a gross SCP.
My favorite SCP-001 is The Factory.
Finally, My favorite one on the server is SCP-457.
I love the classics too, All of the ones that are in Containment Breach.