What's the best Realm to be on for EU players and why?

That highly depends on what your priorities are. High or low population? Active PVP and/or PVE community? RP? Alliance/horde ratio? Etc.
Been playing on Kazzak for many years myself due to the faction ratio favoring horde (which I play), and considerably high horde population. Only surpassed by Draenor and arguably Stormscale.

This might be of help to you, if faction ratio and/or realm population matters to you: https://realmpop.com/eu.html

Just looking for a bit of everything really. PvE, raods, dungeons. Just starting again after not playing for so many years so starting over again really. Like, no one on my friends list plays anymore so will be looking for a friends and guild etc etc.