The Adept class in Mass Effect 2 is the most effective for those who like strategy in combat.
Playing through both the first and second ME games several times with different classes, it is safe to say that the class with the singularity ability is always very satisfying to play. In the case of ME 2, this ability is in the adept class. Combine the singularity with other forceful specialties like warp, push, or pull, this class is ideal for strategy in combat.
In my opinion, ME and ME2 are games that are best played when thought over and panned out, including in combat situations. Even as a soldier class, just running into a fight and shooting up the place doesn't get you very far, especially on the harder difficulties. The adept class abilities help you keep your distance from your enemies, giving you time to plan moves and helps keep you from being overtaken by a swarm, which is very handy when fighting enemies like husks.
The down sides to being an adept are the weapon and armor limitations, for sure. This class can only become specialists in pistols and wear light armor. However, once you begin to level up the abilities of the adept class, they start to greatly outweigh the limitations. Besides, you can eventually acquire light armor safe enough and pistols strong enough to suit your needs quite nicely. Plus, later you have the ability to acquire an extra talent. I highly suggest the warp ammo ability, which will aid with the warp ability in taking down heavily shielded/armored enemies.
Early on it may be tough to play as the adept because of the weapon/armor limitations. Because of this, it is imperative to level up the the most useful abilities of the class, the aforementioned singularity and warp. Once you've dumped in all the points you can toward leveling up these abilities, you will also see that there is a special type of final level for both. For the warp special, I suggest unstable warp.
The unstable warp does maximum warp damage with the added bonus of detonating another biotic's special ability. For example, if you use unstable warp against a biotic who has singularity, this ability will trigger and suck any surrounding enemies into the vortex. The second option is heavy warp which increases the damage of warp by 40%. In fights further down the road it will be clear why unstable warp is a better choice since there are plenty of biotics to face.
For maxing out singularity, both options are actually pretty good. I suggest wide singularity as opposed to heavy singularity though. Heavy singularity will keep enemies in the swirling vortex longer, ultimately doing more damage, but the wide singularity upgrade drastically increases the field of the singularity. This is extremely useful against swarming enemies, like the husks, who may seem to over power you with sheer numbers. With wide singularity you can immobilize a larger group, leaving fewer to fight off.
The secondary abilities are also important. As many enemies throughout the game will hide behind cover, you will learn that pull becomes very handy. You will notice that when you've locked on to an enemy, if you adjust your camera angle/aim a bit you can arc your biotic attacks over or around most cover. Using pull can bring an enemy out of cover to attack. Push is also useful against enemies that are near ledges or other drop off points. You simply throw an enemy over a high railing or off of a bridge and you don't have to waste your ammo fighting them anymore.
Even after all that has been said, the main key to fighting as an adept is to choose your squad accordingly. The adept has no specific combat skills and no specific tech skills. So, it is important to fill your team with others who have training in these areas. A good combination here is Garrus and Grunt. Garrus is great in the tech field with abilities like damping, decryption, first aid and electronics. He also specializes in assault rifles and sniper rifles which is a good pairing with your pistol specialization. Grunt doesn't have nearly as many tactical abilities as Garrus, but he is basically a living tank. He's extremely strong and can use shotguns and assault rifles. He can take a lot more damage and can be sent into closer combat leaving you free to fight at a further distance and apply your biotic specialties. There are other squad members that can also fill in gaps for the tech and combat skill set. Using the right combinations will get you an ultimate win against those nasty collectors!