Any Skyrim player looking for extra challenge or enemy diversity should definitely give the Skyrim Monster Mod a try. Formerly available at the nexus, it is now available at VGUNetwork and Mod Database
For Monster Mod fans looking for even MORE challenge and/or additions, try out Monster Wars, a Monster Mod addon that adds new NPCs, new unique enemies, and tons of new loot!
The same author of Monster Wars also has a Monster Mod addon that adds unique drops to many of the monsters added by the Monster Mod
Finally, there are two MASSIVE weapon pack mods, Nexus Community Weapon Overhaul and Immersive Weapons
Immersive Weapons has already been listed. As for Skyrim Monster mod, I'm having huge doubt about including it here. Notably because how not lore-friendly it is. However, these models are absolutely impressive. Some of which could be intergrated into Skyrim, like female Falmers or Sea Dragons using the Ice Wraith animations.
I guess it deserves a mention, and as for the final topic we'll just see. Personally I do not like it, but if you like to include it, go make a post following this topic format:
(BOLD)<insert mod title here>(BOLD) [mod link]
(insert a short description of the mod description here)
Credit will be given on the post.