When I transferred my fiance and I looked into high pop realms, we chose Area 52. We came from a dead realm and wanted the opposite, we went the extreme opposite and now regret it lol. Don't get me wrong 52 is a great server, but it is extremely populated. When WoD launches it will be hell. However there is always something to do, always even now when the game is "dead" there is plenty to do. The economy is great, loads of guilds. The downside is really being too much of a good thing, there are simply too many people.
That's just my experience. I would recommend checking out realmpop. Pick a few servers that fit what you are looking for, roll level 1s on those servers and ask people in chat their opinions.
You will get some troll answers, but honest ones as well. Plus you can see how the player base behaves, the economy etc.
EDIT: My experience reflects Area 52 US. Realmpop has both US and EU server data
These are the only Horde realms I've been on and I think both are good. Tich can sometimes come with a queue to get in, but it's a very active server that offers tons of PvE and PvP things to choose from. Kil'jaeden is the same, only not as populated, but is still a high pop server (I think it's number 6 on the list)
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Hers a good website that offers the total populations, Horde to Ally ratios, total classes and races played.
Thanks for the advice and what makes you regret transferring there cause i've only seen good things about area 52?
It really is a great realm. My only concern is the server stability come launch time. Long ques, lots of lag, lots of competition for quest items, mobs etc. If that's not your thing I would suggest something with a slightly less population. I don't mind the competition personally, the que doesn't bug me either. But the lag can be incredibly frustrating at times. Maybe regret is too strong of a word, as I don't think I would leave even if it were free and I could have my pick of realms to go to. But sometimes it can be very frustrating.