As others have said Broken Steel expands the game the most, being more of a full expansion pack than an add on. It adds 10 new levels of character advancement and continues on past the end of the game.
Quality wise Point Lookout is the best, and will also probably keep you involved the longest, however it arguably has the least in terms of actual character benefits as far as gear and traits.
The Pitt is the runner up, it's not as fun to explore as Point Lookout (being a giant scavenger hunt for Ore for the most part) but has some very nice loot and perks, especially if you like close combat.
I consider Operation Anchorage to be the least of the add ons because while it provides some interesting loot and an early way to get powered armor training, I found it to be VERY linear and to not take all that long. It just wasn't that much fun for me, also given the complete inabillity to backtrack in this one make sure you have a walkthrough to tell you where the Intel cases are to save some frustration.
Mothership Zeta is one of those love it or hate it type things. I thought it was quite fun personally, though it's definatly quirky. This one has a couple of areas you can get locked out of revisiting, so a walkthrough helps, but isn't quite as linear as Anchorage because you've at least got some desicians as to what areas to tackle first and in what order. The loot from this one is uber, but like The Pitt is mostly geared towards a specific character type... namely one who is going to use energy weapons. I consider this to be better than Anchorage, but not as good as the other three expansions.