It is true that Soviet t1 infantry are lackluster, but that's because they're meant to be strength in numbers of the sort with their low cost and take advantage of garrisons if they can, or even be "meat shields" as some call it (well, at least that works in early game). At least Mortar Flak troops, compared to the other AA infantry, well I find em more efficient at killing lightly-armored air units like Rocketeers and lesser planes, and you don't even have to worry about their shots missing at all. And looking at the stats, Flak and Archers share the same HP amount, so they're not that useless.
Unfortunately they're not as useful in late-game cept defense against jets or rocketeers or something like that. And for Conscripts... yea, they're probably not relevant for late game at all cept for garrisons, but by then your enemy would have something to take em out.
As for basic tanks, it's obvious what's the best, but it depends on the category.
-Jaguar for speed and damage. Definitely one of the best rushing MBTs, and the desperate driver adds to damage.
-Bulldog for speed and harming infantry. Also good at rushing.
-Qilin for durability and damage. Also is still relevant in late game scenarios given its power, and certain matchups actually do call for more Qilins over Nuwas.
-Bison for endurance (if it stays still, which is good against bases without too much DPS)
-Mantis for speed, rushing, and is cheap to build. I almost didn't want to put this one honestly as I find Bulldogs probably superior.
As for the others, well...
-Cavalier: Pretty much all-around and basic.
-Kappa: Doesn't slow down when on ore patches and is amphibious, good for naval support. Just don't do the naval part against PC or China (borderline Russia).
-Rhino: Nothing special about it. Just fairly durable with some power and late-game paradrop, but Jaguars are faster and have that same damage.
-Opus: Double power if infantry inside, can also be used as transport (a quiet one too unlike Stingers)
-Lasher: Only good for hit n run, but they still fall against the stronger MBTs.
-Draco: Its speed boost from spinblades and flying drone feature is great and unique, but its power is lackluster and repelled if there's AA support.
-Cyclops: Fair speed, power, and durability, but good power only if it's close to a target, but that means going right into a standard blob of units (mostly). Great for busting unprotected miners though, but even then they're expensive.