While official Aion servers have moved to 7.x and beyond, the 5.8 client is the most emulated and stable version among private projects. Nearly all high-rate, mid-rate, and “classic” servers offer 5.8 because:
The 5.8 client introduces the region of Lakrum, a map designed to encourage open-world PvP. aion 5.8 client
Recommended client-side modifications for stable play on a 5.8 emulated server: While official Aion servers have moved to 7