We spent a few days in Saigon and since I'm clearly suffering from some sort of brain lesion, a fine chunk of that time was used to track down and obsessively eat a single food: phở.

Phở from Phở Anh.
Much like with xiao long bao in Shanghai, we sought out the most famed, the oft recommended and the randomly spotted, then pitted their fare against each other in a battle to the death. (Note: very few people actually died.)
I'm confident that most of you are well acquainted with the concept of this particular soup so I won't bore you with an in depth diatribe about its origins, history, development, height, weight, and sexual preferences.
Instead, I'll give you a brief rundown on the criteria and rubric we used to make our assessments:

I. Broth:
Phở broth is the biggest part of the show. It's the area that allows for the most variation, establishing depth, character, and giving the soup a chance to knock you on your ass with delicious. Since we were in Ho Chi Minh City we were looking for southern style phở, darker, sweeter, and with more garnishes and floaty bits.
a. Every phở broth is proprietary but basically they're all made from simmered beef bones (roasted or not or some combination), other cuts of beef or oxtail, fish sauce, and various vegetables and spices that can include onion and ginger (charred or not), cinnamon, star anise, clove, coriander, fennel, and black cardamom (all toasted or not).

Phở from Phở Ngân.
The most important part here is balance. all of the constituents should support the whole, with no single flavor or ingredient dominating the bowl.
b. My favorite broth has a medium level of oiliness. Enough to give it richness and body, but not so much that it requires chemical dispersants. (Note: If you're a particular lover of beef fat you can order a bowl of nước béo (extra-fatty broth) on the side.)
II. Noodles:
This one is easy as almost every phở place uses the same style of prescribed rice noodle (called bánh phở, or phở noodle, cleverly enough); it's all in the prep and presentation.

Phở from Phở 24.
a. Not too many, not too few. The noodles should take up somewhere around a quarter to a third of the bowl.
b. The damn things shouldn't be in one big clump. If you can't cook noodles I don't trust you with the rest of it.
c. Not too soft, not too firm. Phở noodles shouldn't be mushy or gummy but they should be a bit softer than Italian style al dente.
III. The Meaty Bits:
All of the phở we ate was phở bò (beef) not gà (chicken). In the interest of consistency (and also because it is the most awesomely-bestest choice available) we ordered all of the soups đặc biệt, which means specialty.

Options, options, options...
a. With đặc biệt you get all or some of the following: tái (sliced rare/raw steak), chín (sliced well-done steak), nạm (flank steak), vè don (crunchy flank), gầu (brisket), gân (tendon), sách (tripe), and bò viên (beef meatballs).
The steak should be flavorful and tender; the brisket should be moist, not dry and mealy; the tendon should be soft like a savory jujube, not gross and crunchy like a piece of gristle; the tripe shouldn't be too funky; and the meatballs should have a pleasant sproing and squeak a little on your teeth.

Phở from Phở Hòa Pasteur.
PROTIP: No matter what kind of phở you get, always order hột gà on the side. Hột gà (just means chicken egg) will get you a creamy yolk that gets dumped in the soup and softly poached by the broth. It brings the phở experience to a whole new level.
IV. Garnishes:
In the south they like their phở with plenty of stuff to mix in. The more variety the better really.

Phở from Phở Khoa Thu.
a. Herbs: A good phở place should present you with a glowingly fresh and crisp plate of vegetable material. Look for: Holy basil, cilantro, scallions, saw tooth coriander, bean sprouts, sliced chilies, and lemon or lime wedges.
The bean sprouts should snap; the herbs should be fragrant and unwilted; the citrus should be glistening with juice and the chilies fiery hot.

This is the face of a man pleased to serve you phở.
b. Condiments: Three condiments should be in evidence- nước mắm (fish sauce), tương đen (hoisin), and tương ớt (chili sauce). Ideally, some of them will be house made. At the least they should have vetted some excellent brands. A phở place with shitty condiments is gonna have shitty phở. End of story.
These are the places that we tried and what we thought about them:
Phở 24 Score: C
271 Phạm Ngũ Lão St., District 1, HCMC
Summary: This place is a huge chain with many locations around town. When we were here in 2007 we had a great experience. Sadly, this time around it was pretty forgettable; thin broth with minimal spicing. Meh.
Phở Anh Score: B+
8-10A Ky Dong St. (near Le Van Sy), District 3, HCMC

Summary: Phở Anh has very good soup. Medium oiliness; nice spice with a fragrant cinnamon smell and a touch of black pepper; rare tender beef. Fresh chilies were dried out and the brisket was a bit tough.
Phở Hòa Pasteur Score: C+
260C Pasteur St, District 3, HCMC

Summary: This place is a Saigon institution. It has great ambiance and a friendly staff. Their herb offerings are fresh and generous and they make a mean lime soda. Unfortunately, the phở itself lacked depth, the cartilage was crunchy as a jawbreaker, and the meat to noodle ratio was all off. Also they over blanched their beansprouts.
Phở Ngân Score: A+
258B Pasteur St., District 3, HCMC

Summary: Phở Ngân was a dark horse candidate. We were drawn here after seeing quite a bit of chatter in Vietnamese on Foursquare. Man am I glad we did- these guys knocked it out of the park. The stock is redolent of star anise and other sweet spices; it's rich and balanced with bottom, middle, and top notes all playing their proper support roles. The accouterments were lovely and they even had great iced tea.
The kicker though was the brisket. I'm normally not the biggest fan but the slices in this phở were so good that they reminded me of some of the best bbq I've had in Texas, you know, except boiled and in soup. The gelatinous tendon was also tops of all the ones we tried.
Phở Khoa Thu Score: A
Head down Ben Van Dong Street in District 4 and make a left onto Nguyen Khoai Street. The stall is located 300 meters down the road on the left hand side.

Summary: We found this place based on a recommendation from our friends over at Gastronomy Blog. Thanks guys!
Khoa Thu wins hands down for most fun place to have phở. The outside setup is charming and inviting and the people that run the place couldn't smile more.
It doesn't hurt that the soup is damn fine as well. Quite heavily spiced, it is also the sweetest phở we had. It has an excellent beef and star anise flavor. Whatever you do, don't miss out on their house made chili sauce. Hot and garlicky, it's perfect to keep on the side and dip pieces of the meat in separately.

And as a bonus, this adorable lady is set up to squeeze you glass after glass of cool and refreshing nước mía, or sugarcane juice.
I hope our little taste test adds to your fun the next time you're in Saigon. And remember, if you don't agree with any of our assessments, you're just wrong.
For more info check out these other very well done guides to phở: One on ordering and one on pronunciation.