Technically, each general has his strengths and weaknesses. Over here, I shall post a brief analysis of the play style of each:
USAF: King Raptors with lasers can each deflect the worth of one firing patriot missile, making base invasion on a turtle easy, they also have high damage, awesome reload, and low prices, making them convenient as a main force. Carpet bomb ability without even getting a promotion can be decapitating. However, a mass of quad cannons takes him down, as he has no heavy tanks to counter. His AI is strong if given time, but poor during the early game without a stable economy.
Rating: 8
USSW: Particle cannon reduction cost makes it easier to obtain them, and her EMPs can shut down tanks, however she lacks a main tank force, and can easily be ousted by a rusher. Her EMPs have low damage, and leave no rangers. Her AI, to the new player, is overwhelming, but not to difficult against a seasoned player.
Rating: 7
USLG: Although this general does not boast many changes, he does, in my opinion, have the best defense system in the game.It can target air force, infantry, and tanks with ease, and can even shoot down SCUD and tomahawk missiles, and then target the vehicles firing long-range-projectiles. His tanks are excellent, but shut down when there is no power source. Invading him is extremely difficult if he employs proper defense. He has an average, slow AI.
Rating: 8.5
CIG: This is a very tactical general,since he lacks a main tank battle-force, missing out on all mighty overlord. His helixes, troop crawlers, and assault outposts pack a punch, and his mini-gunners can raze airforce with absolutely no difficulty. His bunker is extremely durable, and can survive a particle beam in motion. However, he lacks firepower and armor to challenge other generals sometimes. He is excellent during early game. His AI is capable, but stands little chance versus the others due to poor infantry management.
Rating: 8
CNG: Although seemingly like super weapon, he has a tank force, as well as enhanced power. His fast tanks that leave radiation, explosive migs, and automatic obtaining of nuclear lobs make him one of the most well-rounded generals. His AI is usually extremely strong, especially in late game.
Rating: 8.5
CTG: This general's tanks are capable of ripping apart most arsenals, and when equipped with the faster fire rate upgrade, create a superforce combined with emperor overlords. However, it is difficult for this general to break a tough defense, due to lack of ranged weapons. If given time to grow, an overlord rush can be unstopppable. His AI is the easiest to beat, due to simple strategy of tanking.
Rating: 8
GLADG: This general is also extremely well rounded, missing out on no tanks. Also, his scud launchers and scud storm sacrifice toxins for high explosives, a worthy trade. His SCUD storm can eliminate super-weapons with one launch. However, his main weapon is his ability to detonate any unit, rendering workers units that can defend a base, and making his infantry difficult to roll over. A rebel ambush can be critical due to demolitions ability, and can take down almost any building. As with all GLA AI*, he is overpowered.
Rating: 9
GLASG: With invisible bases, this general can be tough to find, however he is easy to beat. He lacks firepower, due to the absence of scorpions, marauders, and SCUD launchers. Attacking him is easy, but annoying due to the strategic possibilities of stealth units. His units are capable of turning the tide of the battles, as he possesses stealthed sabatours (at all times) and when he captures another technology, the combination of stealth, sneak, and tank can prove devastating. His last buildings can be annoying to find, and one will need to force fire defenses sometimes. His AI is good, but does not compare to the other GLA generals.
Rating: 7.5
GLATG: Thrax properly puts use toxins to create a mess on the battlefield, as infantry stands no chance against him. His toxin rebels are more powerful, and can challenge tanks, and even buildings when massed. He is well-rounded, but lacks Demo's explosive touch, making it difficult to wreak havoc with him. Excellent AI can be played, as with all GLA AI's.
Rating: 8
I hope this serves as a future reference when determining the best general, although the ratings are from my perspective.
*Due to an unknown cause, GLA AI in zero-hour is extremely buffed up, and can take down any other general on a one vs one no matter which map is utilized.