Last night I did the Battle for the Undercity on Alliance side for the first time, which inspired me to write about this quest in general and why I think it's overrated. On Wowhead, comments like "best quest ever" get rated up, and anyone criticising the quest seems to get downrated into oblivion quite quickly. I'm honestly not sure why that is.
Now, to clarify, I did rather enjoy the battle the first time I got to participate in it. The quest does have three major points going in its favour as far as I'm concerned:
1. Revolutionary use of phasing mechanics. In the game's current state, the Wrathgate and the following hijinks at the Undercity are the earliest points at which you can experience phasing in the game, and like most new technologies it's quite shiny and impressive at first. But if you continued questing until Icecrown, you'll have experienced many more quests along those lines by then, some of them even going as far as to create whole new outposts with associated quests and flight paths for the Argent Crusade and the Knights of the Ebon Blade. You might also have got to experience the problems that currently exist with phasing, such as being unable to help your friends if they are in a different phase or struggling with inconveniently placed summoning stones. That kind of thing definitely tarnishes your intial impression of "that's so awesome" after a while. Also, in Cataclysm phasing will probably exist all over the place, so seeing it in action won't be nearly as exciting anymore.
2. Getting to fight alongside important lore characters. This is kind of dependent on you having some kind of positive connection to the characters in the first place - for example I just rolled my eyes at Varian Wrynn the whole time - but assuming you do care, it can be quite a rewarding experience. I rather enjoyed Sylvanas' song, and seeing Thrall call for the help of the elements to clear a path for the Horde forces made me appreciate why he's the warchief so much more. Unfortunately WOTLK in general has overused the "interact with important NPCs" gimmick in a lot of respects, so this is another thing that significantly goes down in value after you've quested your way through all of Northrend once.
3. If you're a fan of seeing big numbers when you press a button, this quest caters to you. Personally it doesn't do much for me, and I got tired of spamming my AoE button after three minutes at the latest. I dread to think what this quest must be like if you do it for example on a rogue before you get fan of knives. Dreadfully slow, that's what.
Now, I already added qualifiers to those three points, saying that there are factors that will decrease their positive impact. However, there's also one big negative that is always present and which is the main reason why I'd never label the Battle for the Undercity as WoW's best quest ever: the fact, that game-play wise, you're actually not doing much of anything.
Sure, AoEing everything down while laughing at the big numbers speeds things up, but the NPCs by no means need your help. I'm pretty sure that if you wanted, you could just go AFK for half an hour once you started the quest and wait for it to complete itself. What this means is that it's in effect the most tedious escort quest ever (commenters on the Horde version clocked it at about three quarters of an hour while participating; the Alliance version is a bit shorter), as you follow your faction leader around while they do stuff they don't actually need your help with. How could anyone consider this a great idea for a quest?
I was actually grateful that the Alliance version seemed considerably shorter, including only one mini-boss before killing Putress, though the fights seemed to be tuned for maximum annoyance to make up for it. For example the mini-boss, a so-called "blight worm" grabbed and stunned me every couple of seconds, making it impossible to get anything like meaningful dps in. Similarly Putress summoned an endless supply of weeny adds that made it impossible to get an uninterrupted cast off, meaning that I even had to spend the last boss fight spamming nothing but AoE while waiting for Varian and Jaina to actually finish off the big guy. I don't remember the Horde's fights being that annoying, except for the second mini-boss having somewhat fiddly aggro.
I also came to the realisation that Alliance and Horde definitely get seperate phases for the quest, as the scene in the throne room plays out slightly differently depending on which side you're on.
All in all I'd say the Alliance version is better if you just want to get it over with quickly, while the Horde version has a bit more lore flavour to it the first time around, even if it's annoyingly long. Either way it's a quest with an interesting story, but as far as your actual contribution goes, I still find doing ten "kill some of these mobs" quests more engaging than following Thrall or Varian around for half an hour. They might as well have made it just another cut scene.