USSR (Best for noob players, good armour good guns and the hand of Stalin
OBj.277 and 705A are very good. The T-100 LT used to be the best light tank in the game until the arrival of French clown cars, so you may want to skip it. You may want to go for the Obj. 430U and not the K91 because it's OP, plain and simple. K91 is hard to play and not very fun (unless you are a very special type of player or a bored unicum). ST-II is meh and a waste of time, effort and credits IMHO. Obj.140 and T62-A are somewhat outdated and outclassed by the Obj. 430U. They are very mobile, have very good accuracy, very high roof, low alpha and lacklustre gun depression. The best Russian medium in their style would be the Obj. 907 (a CW reward tank).
USA (Hull-down monsters with ok grinds. Not very noob-friendly, you should know what to be doing)
T110E4 (turreted TD) is way inferior to the T110E3 due to its much weaker armour, so I would go for the E3 and forget about the E4. The lights … well, EBR 105. M48 Patton is a great tank, a bit on the slow side, but with a good turret and an awesome gun. T57 HT is a very strong tank, too – soft turret, but very fast reload and brutal premium penetration if you need it. The T110E5 might be worth grinding again if it will indeed be buffed as proposed by WG recently (which currently is in a super test state, so not necessarily final).
CHINA (Tough stock grinds and Russian plagiarism but the fruit is sweet)
The only tank worth grinding here is the WZ-111 5A due to its great mobility and DPM. The TD is horrible, the med has been forgotten by WG, and the light … well, EBR 105. The 113 is a bit like a better armoured 121: Both suffer from a lack of gun depression and have a huge hull that is easy to damage at the sides. It was great in its day, but that's a while ago, and it is vastly outclassed by the WZ-111 5A.
Not-so-Great Britian (DPM or Damage Monsters meh)
The light … did I say EBR 105 before? Btw, it is by far the worst tier X light tank in the game. Grind it if you are in dire need of a challenge. While the lower tier TDs are mostly fun to play, the tier X TDs aren't exactly meta either. You may want to grind the FV 4002 Stage II (aka "Shitbarn") if you have everything worth grinding and are looking for some diversity. The Super Conqueror is one of the best heavy tanks in the game right now. The Centurion AX has gathered some dust but is still very playable due to its great gun, good gun depression and high penetration HE ammo.
FRANCE (Not for noobs)
The EBR 105 is a must-have right now. The TD is so so (I wouldn't waste my time grinding it), and the single-shot heavy tank is crap. AMX 30B is more or less a worse Leopard 1. What you really want to grind here (if you can handle that type of tanks) are the Bat. Chat. 25t and the AMX 50B. The latter may lack turret armour, but the mobility is outstanding and the gun will rarely fail you. The AMX 13 105 is really good for its manageable clip size and speed but EBR exists.
GERMANY (Largely Outclassed, either hug walls like a koala or kemp bush)
The Rhm. Pzkpfw. used to be the worst tier X light tank in the game until the advent of the Manticore … Nuff sed. The grille is great. Pzkpfw. VII is grossly UP due to the major weak spots under the turret pointing forward. If you want a German super-heavy tank, go for the Maus, which is still very playable (hint: The Maus' aim time is very fast, so you don't need a stabilizer or rammer – use a super heavy spall liner instead). The E50M is still a great medium tank IMO if played right, and unlike the Leopard 1, it has some armour on top of its incredibly reliable gun. To mention the iconic E-100: Even with the proposed buff it won't be good enough, because the "effective" armour of 311 mm as shown on the screenshot from WG are just 270 mm standard armour – the "effective" armour depends on impact angle and normalization, meaning it will be significantly less than 311 in the majority of cases. So sadly it wouldn't be worth the grind.
JAPAN (IMHO MT Line not easy to play and HTs were nerfed)
STB-1 IMO is the best medium tank in the game right now. It is very mobile, has insane DPM, a very good gun and up to -14° gun elevation due to its hydro-pneumatic chassis, combined with a pretty hard turret. Its major weakness is the very thin engine deck (20 mm) which means that arty hits there will penetrate very often and do major damage (1000+). So you really want to stay arty safe with it or keep moving when there is (a lot of) arty in the match. Type 5 Heavy has been nerfed to death and is more or less garbage right now.
ITALY (Sphaget ned brains not for noobs)
Progetto 65 is extremely strong and a must-have tank IMHO but you have to have some game sense and when to shoot and when not. Rhinoceronte is a classic WG Failure with crap DPM meh armour and an annoying beep in magazine reload which surpasses all its bad traits and makes it plain annoying. Avoid
**CZECHOSLOVAKIA (**It's a damaged modules party and I'm winning! Not for noobs)
TVP T50/51 is a beast. Its major drawback is the bad gun depression, but you can dispose of the clip super fast, meaning you can risk going on ridgelines to kill a target, and it has a very fast reload. IMO, the TVP T50/51 even is the "better AMX 13 105", if you are looking for the French scout …
SWEDEN (Bit difficult to play and each one made for only one purpose so isn't that flexible)
All three tank lines are great, although the TD may be a bit hard to play because its siege mode mechanic makes it a little unflexible (nothing you couldn't compensate with a little practise and a sense for the battlefield and flow of the game).
POLAND (The stock grind shall kill if you live to make it till the end, you're situational, enter at your own peril)
For the sake of completeness: The Polish heavy tanks are pretty good until you arrive at tier 9, where they start to be rolling weak spots. Unfortunately, this is true for the 60TP too, which makes it somewhat … delicate to play. The guns are absolutely brutal, tier for tier. You don't want to cross the business end of a 50TP or 60TP gun unless it has just fired, but when it did, they are rather easy to damage (60TP: Turret ring). So IMO they are rather something for completionists or players in need of a challenge. Stay away from CS-63. The unfair gun handling in Turbo mode will kill you when EBRs are STRVs at 95km/h
BOND SHOP (Waste of bonds)
IMO, all tier X tanks currently in the bond shop aren't really worth the bonds. The M60 has a very weak turret, the FV 215b has the same type of weak spot under the turret as the Pzkpfw. VII and the 121B is very much out of the meta. So they are probably rather something for completionists and people with too many bonds.