I generally prefer the Allies. In multiplayer I usually use the Germans or the France. The allies require more skill to master than the soviets because the allies use advanced technology while the soviets use brute force. Thereocitally, that means if one is a great commander and knows how to use the technology well, the one will dominate in multiplayer. The first thing an ally commander must remember that GIs are powerful. Sure, a lone GI will get his ass major whipped, but say oh, about twenty GIs deployed will do tremendous amounts of damage (I had to learn that... the hard way). Also keep in mind that although the Grizzly tank is less powerful that the Rhino tank, it is much faster. You also have to watch out for the damn Apocalypse Assault Tanks. That mamma's one volley can take out a prism tank! I prefer to destroy it from afar, say Grand Cannons backed up by GIs. Sending tanks to assault them are suicide. GIs deployed and long range prism deployment is the best. Another thing to watch out for is the dreadnought. It is extremely powerful, but if you are a good commander, you should have a big group of rocket IFVs guarding your base for anti Kiev Airship attack. A big group of those should be able to destroy those missiles while your carriers, destroyers, harriers, Rocket troopers or dolphins can go take them out. One thing that is very useful is Rocket troopers. They don't do much damage so there is no point in building a lot of them. Besides, they are expensive. But they are great for scouting and helping with ambushes. The harrier is a great unit. If the enemy neglects to build air defense, punish them with harriers. Harriers are also great defense, because when verging allies, they usually don't view the air force as a threat, so they send little anti-air units in. Use harriers to take out their more expensive units. I.e.. prism tanks. Speaking of prism tanks, the prism tank is a great unit. It has decent range (longer than any tank), and does great damage. It is also great for defense and offense. The only drawback is that they are too expensive. Basically to sum up, a good Grizzly/Prism force with lots of deployed GIs can win your ground battles.
You know what they say... a good defense is a good offense, unless you use stationary structures. I generally don't like to use stationary structures unless it is the Grand Cannon or there is great tactical advantage due to the placement of the structure. Since I am usually Germany, my defense usually consists of a hell lot of deployed GIs, in a claw sort of formation in the front, all spread out to prevent vehicle squishing. Then I will have a mobile Tank Destroyer/Grizzly force to help the GIs with tanks. I might throw in a Tanya IFV for anti- infantry. Then, behind that GIs, I would have prism tanks. This is a great formation because the enemy will tend to go for the prism tanks (which I have like 2 repair IFVs backing them up), and the prism tanks have decent range and the GIs will usually tear the enemy to shreds (with the help of the long range prism fire) and with the help of my Tank destroyer/Grizzly force. I would also have a armada of rocket IFVs for anti missile/anti air defense. They work great, they are fast and destroys missiles and air like mad. However, the bulk of my defense will still be my GIs. I learned this great defense tactic in a multiplayer game. I was playing a 3 player free for all. The bad player got killed in the beginning. I could have won the game early with a chrono-prism rush but I messed up so I decided to start building my Grizzly tank force. I had about 25 grizzly tank and I saw only (or so I though) 30 GIs deployed with little tank backup, so I decided to run the GIs over and kill the tank and the base is mine. I was wrong. The GIs tore my tanks to shreds before they even got near the enemy tank force. Therefore, I learned this tactic the hard way, and I used it ever since. USE IT!
There is one very cool thing (it doesn't always work but it usually does)
that you can do with the ally chronosphere. I call it the chrono-prism
strike. What you do is you build up a force of about 9 prism tanks, and a
chornosphere. Now, this is a pretty expensive tactic, so make sure you
have backup in case this does not work. The enemy will usually not build a
defense around their construction yard, they will usually concentrate their
defense on choke points (at least I do). When ready, have your prism tanks
go to one flat place and chronosphere them right next to the construction
yard. Tell them to attack the construction yard. The chances of you
not killing the construction yard is like 1%. If the enemy's defense force
consists of moving units, then you still have like a 5 second attack
window. Then direct your tanks for the war factory, so they cannot build
more construction yards or heavy units, the probability of this succeeding is
50%, because prism tanks rule at taking out structures. Then, you can use
your remaining force to destroy some other crap. This will basically
cripple the enemy's building supply. They might try to pump out
GIs/conscripts after this (which is pretty devastating), so have Tanya IFVs
ready. A good defense for this is when you hear a chronosphere has been
built, get a few prism towers inside your base. Or, you can have tank
destroyers standing by to take them out, or like what you should always do, get
an extra MCV. A good tactic that works in the beginning of the game is
that when you set up your GI defense, and you have a decent number or grizzly
tanks (and you have spy sat), try to flank the enemy base. Destroy maybe a
power plant or something, but do not run into the enemy defense. The enemy
defense will attempt to chase you. As you get to your GI force, they will
get slaughtered by GIs, providing your strike force come back and help your
GIs. Another good tactic for later in the game is as you have a huge tank
force and so does the enemy, it is preferable that you have tank destroyers),
you go with a lot of GIs. You tanks will engage the enemy tanks, while
maybe the prism tanks will cover the GIs (anti prism tower, teals coil, pillbox,
etc..) and have your GIs storm the the center of the enemy base and start firing
at the enemy's key structures. The enemy tank force will have no choice
but to retreat and smash your GIs, but at this point, you should have had a few
key structures down. Then, have the GIs engage the tanks, it will be
pretty hard for the tanks because the enemy structures are in the way.
While you do this, your own tank force should attack the enemy tank force from
their side, a two sided attack on the enemy defense force. Or if you think
your GIs can take it, take out some buildings. The Ally offense can be
very devastating, providing you use some brain power.
The allied naval power is nothing you should poke a stick at. It is one of
the most powerful aspects of the Allied force. One very important unit that
you'll need is the dolphin. The dolphin's stealth and speed will make it
very important in your navy. I usually have a force of dolphins to keep
the enemy naval force under siege, while I have aegis cruisers to protect me
from air and V3 missiles. Then I use my aircraft carriers to bring down
the enemy ground force. Although I did not incorporate destroyers in this
attack, they will be a very useful support unit. And also, since you can get
them early, it is best to use them for defense while your other units
attack. Again, when possible, build up a fleet. 
Prism Tanks is an awesome offense ^
The GI fortifed defense with mirrage tank, prism tank, grizzly tank and tank
destroyer backup ^
Hmm... The image looks familiar... yes it's like the carriers in Starcraft, the
Ally Carriers rain death upon the enemy ^