Which unit on soviet side that better than OST?

Stock unit only.
need some suggestion for grinding soviet ranking.
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120mm Mortar.

LMG Guards and Shocks do their jobs better.

7 man conscript doesn't really have a comparison. Its really nice.

Lastly, the T-70 has no contender. there's a reason its mandatory in 1v1.

Understanding each units role will yield better results probably. Saying the p4 is better in 1v1 vs a t34 will make you shy away from the t34, but the t34 is great in its own right of being cheaper and haveing superb AI damage with its MGs.

But if you're looking what soviet unit to just spam out, 7 man cons are always good.

Do you mean what OST unit would be better on the Soviet side than OST then I'd say P4.

Combat engineers are slighty more cost effective after they get flamers

T70 is pretty decent

and SU85 seems more cost efficient then Panther if you dont play on CQB heavy map

Also flare is cool

It's a bit tricky to say, considering how the faction is designed. Most Soviet units, especially the basic factional ones, are designed as generalists: they do okay at a wide range of tasks but don't really excel at any one thing. In the end, it really comes down to how you use them, not how they perform against other units.

Conscripts for example, are utterly horrid at infantry firefights, but their large squad size, inexpensive costs, and wide range of abilities allow them to fulfill myriad roles within whatever force composition you've come up with. They're great at holding the line, what with their large squad size and sandbags. With Oohrah, you can also use them to rush and flank enemy units, either to drop a Molotov on a team or snare a tank with AT grenades. Merge is a good way to save manpower and keep more important units in the fight. Mobilize Reserves makes them better at what they already do, while tacking on a fairly substantial DPS boost to boot.

That said, there are a few standout units that are probably the best in their niche:

T-70 is the best light vehicle, hands down. The combination of mobility, armour, and firepower is potentially unbeatable considering the timing you can get them at. They'll beat pretty much any other light vehicle except the Puma, which has much stronger AT but is much worse at everything else. It also has great late-game utility with its Recon Mode, which you can use to scout for your mortars and other artillery, and after being vetted, it allows you to cap points without infantry, which is great for policing your own capture points or sneakily grabbing a lightly-defended enemy capture point.

Katyushas are the best artillery unit in the game. Effective against emplacements and infantry, reloads pretty quickly, very mobile, and has the longest range to boot. Not too expensive either. Its main fault - and it's a pretty big one, at that - is that it is very fragile and won't stand up to any fire at all. It also tends to be rather inaccurate with its regular saturation attack, but the fact that each barrage is fired in four waves makes it pretty good at area denial.

I also think that, for its cost and abilities, the SU-76 is definitely one of the more underappreciated units in the vaguely-defined light vehicle category. To be sure, it doesn't really specialize at anything, but it does a lot of things reasonably well: fighting other vehicles, splashing infantry blobs, blasting teams and fortifications, and so forth, thanks to its long range, decent penetration, and barrage ability. It's not exactly durable, but it can at least resist bullets better than most vehicles at its tier, so there's that.

If we are to look at non-doctrinal units though, then you have Shock Troops (the strongest close combat troops in-game, with few rivals), 120mm mortars (best overall mortar for blasting blobs, but actually surprisingly limited in other areas, given its reputation), and the DShK (worse overall at anti-infantry than the MG42 but compensates by being harder to kill and just a bit more flexible overall).

More generally, I also think that the Soviets have the strongest medium tank roster with all bases covered: they have access to cheap mobile flankers (T-34/76), a solid tanky-tank option to rival the PzIV and possibly the Panther (T-34/85), and a superb generalist that can handle pretty much any role you throw at it (M4C).

T34/85 Tanks.

They are freaking monsters.

What does playercard have to do with this?

I'm surprised nobody's mentioning cost-effectiveness of 7-man conscripts.

It is a tricky question because the are very few overlaps between factions.
If talking about stock and units that overlap and are clearly better:

OST
HMG42 >> Maxim
Grenadiers (early-mid game) > cons
PGrens >> penals (kind of a stretch though, because of the timing, although these units share some similarities)

SOV
Katy >> Pwerfer
7men cons >> Grens
SU85 >> StugG