The Horde and if a mod named CantFaketheFunk tells you otherwise tell him to fuck off right back to the Twisting Nether!(no offence to CFtF ofcourse

)
The Horde have a better story, with three of the races being honourable yet savage and only seek to want a place in the world of Azeroth.
The other part simply wants revenge and the Sunwell (even though they have it now and can stop bitching) revenge for what? Aside from the obvious being turned Undead the Forsaken just seem generally pissed at everything which is also very cool.
The Alliance however is a group of self-righteous morons who end up doing more harm then good, the bearable ones are the dwarves because they're imperialistic and drink alot but then I remember they are related to gnomes and by kinship all need to die as well.
This is one point no one can challenge me on:
What Alliance hero can challenge a Horde hero? You can't say Malfurion or Tyrande about saving the world twice because they did that as a part of a neutral Night Elven race.
Like:
Thrall > Varian
Saurfang > Bolvar
Grom > Uther
Garrosh All of them (Dude jumped into the twisting nether and saved the world and a cut into the leg of the ultimate evil)
L80ETC > Stormwind Polka Band
(Jaina also doesn't count as she is part of the separate Kul Tiras faction which only ever aided the Alliance)
Edit: Oh I got all unnesseccarily lore-y, my points still stand. But Funk's right the player base is dependent on the server, I've often found that the "losing" faction is often the nicer and more welcoming one (Or atleast on Nagrand because honestly the Allies there are wooting dickholes)