This is my case on why the infiltrator is the best class in Mass Effect 2.
Honestly, I could make a real case for any of the 6 classes in Mass Effect 2 as being the best, but after playing as all 6 of them my clear favorite is the Infiltrator. I'm not saying that everyone will love this class or see it as their favorite, but here are a few reasons why it is mine.
First thing I looked at when choosing the best class was seeing how well it worked on every difficulty in the game. While you can clear the game on the easier difficulties with just about any class, the addition of shields on every enemy on hardcore and insanity make the adept harder to use, due to the fact that biotic powers have little effect on shields. It is true that you can bring squad members with you that can take care of that, but that's not what this is about. This is about Shepard and his/her classes.
Next I looked at the Vanguard class. In Mass Effect 1 the Vanguard was one of the most powerful classes simply due to the power of the shotgun. In ME2, the weapon was given a lot less power, making it hard to utilize if you aren't an expert at using it. Also, while the Charge ability can be deadly if used correctly, it is a suicide move when incorrectly used. Vanguards are great for some, but most will find the class difficult when they first pick it up.
While the Soldier class has the advantage of holding the most weapons, it is disadvantaged by having no special powers (except for Adrenaline Rush). With 5 weapons from the start, you can't choose a new one when you go to the Collector Base the first time. With every other class, you can add a new weapon to your arsenal (the assault rifle is highly recommended since you will already have the SMG). To make up for the lack of powers, you are offered 3 different ammo boosts. While having one ammo boost can be great for combat, 3 is too much. Since you can't use more than one at a time, you are pretty much given 2 blank spots once you pick your favorite ammo boost. This leaves 3 classes left.
With Engineer, Sentinel, and Infiltrator left to choose from, the question is "Which class gives me the most to work with?". The first thing I look at with these 3 is the abilities offered. One of my favorite abilities in the whole game is Incinerate. It is a big blast is fire that burns any organic enemy to ashes. The choice between Incinerate and Cryo Blast is all a matter of opinion, but since this is all about my opinion I choose Incinerate. This rules out the Sentinel class.
This leaves 2 classes left: Infiltrator and Engineer. Both classes have Incinerate. Both classes even offer abilities that can be used against AI (both have AI Hacking). When it came down to it, the ultimate decision on the class I chose was down to one item: the sniper rifle. Infiltrators hold a sniper rifle, allowing for long range shooting, and Engineers do not.
When it all comes down to it, I choose Infiltrator because it gives me a complete package. With abilities that allow damage to both organic life forms and AI combined with the long range ability of the sniper rifle (and the ability to hold both a SMG and assault rifle after the first visit to the Collector Base), both your weapons and abilities are the best combo for any squad leader.