Minecraft Modded Sever

[Truecalico]

• tekkit is always a good one, kinda focuses on machines a lot which can be good
  
• blood and bones would be good for challenge, lots of harder mobs, food dificulties, and just generally everything is death
  
• infinity has a lot of mods (like 160 or so), so there is a lot to do in both magic and machines
  
• pixelmon isn't my favorite , but if you enjoy the Pokemon games, you may like it
  
• also there are tech world and magic world which both focus on a different branch of mods (machines or magic) (both are 1.6.4)
  
• blightfall would be good for a challenge as well, it is a premade world with thaumcraft taint everywhere and the goal is to purify the world
  
• hexxit is a nice basic one, no machines or magic, just exploration and all that
  
• mage quest seems like it could be an interesting one, it is in 1.7.10 as opposed to magic world being in 1.6.4 and has some different mods
  

those are about all i can think of at the moment that might be good