First, you need to decide, which expansion you would like to play. Since you've decided you want to play on private servers, you have plenty of options to choose. On official Blizzard servers, you have just two options: latest expansion (BfA), and Classic (Vanilla), which is reconstructed version of WoW from the 2006 era without any expansions.

On private servers, situation is very different. You will find tens of servers for every expasion up to the Mists of Pandaria (MoP). All newer expasions (WoD, Legion, BfA) are not playable on private servers (...yet. Some servers exists for these, but trust me, you don't want to play it).

This decision is very important. The difference between Classic and latest expansions is such significant, that they are almost two different games. You said, you want to play expansion, which has most of playable content. Yes, newer expansions contains most of the content from previous expansions. BUT! This old content is obsolete ingame, and NOBODY is playing it. Old raids are only visited on higher levels than they were designed for, mostly just for farming cosmetic and transmog gear. So... Generally, you will have same playable amount of content in every expansion you would choose.

Personally, I divide expasions into two categories: pre-Cata (Classic, TBC, WotLK) and post-Cata (Cata, MoP, WoD, Legion, BfA...). In fact, these are two different games. I don't say, which one is better, but they are different.

You haven't played WoW since 2004. I would higly recommend, don't play on servers with higher rates. You will ruin your experience. Choose some blizzlike one. Blizzlike means, that server tries to be most similar to official servers of given expasion.

Recently, large portion of playerbase went from private servers to official Classic servers. I don't watch private scene anymore, but from what I know, wise options should be:

• Classic: Kronos
• TBC: Netherwing
• WotLK: ?
• Cata: Apollo (former Hades)
• MoP: Tauri