As the title says.

What is the best software for adding mods?

I.ex Nexus Mod Manager?

I believe Nexus Mod Manager. It's so easy to use.

It's not the most advanced, but from what i can gather you're not looking for that.

It really depends on how much control you want over your mods, and how much you're willing to learn about making them work together properly. Nexus Mod Manager is as close to a "plug and play" type system that you're going to get (this side of the Steam Workshop, anyway), but it does have its limitations. Wrye Bash will give you a lot more control over your load order and has lots of features to make mods work together, but you have to be willing to invest some time and effort into learning how mods work, and how Wrye Bash itself works.

No matter which you use, you really, really, really need to use https://code.google.com/p/better-oblivion-sorting-software/downloads/list to sort them properly, especially with a long load order.

Wrye Bash

NMM to install the mods and Wrye Bash to move them around in your load order with the aid of the essential BOSS software

Allright. I think Ill try Wrye Bash then.

Want to make sure that I get no problems.

Another question. Is Nexusmods slow for you too? It've been really hard for me to download and even enter the site for the last couple of days.

I've noticed the Nexus has been very slow these past few days myself, and I get quite a few 502 errors when I try to visit or download files. Looks like they need to do some more site maintenance.

Wrye Bash is definitely very recommended tool for managing mods. It has a bit steeper learning curve but it's not that steep. It's however best to learn to use it since at some it'll become mandatory, you'll need eventually make Bashed Patch when merging leveled lists is needed.

Nexus has been slow. Propably just traffic, the Legendary Edition has only been released and more players are likely diving into mods. Nothing we haven't seen before.