Depends lor..

Some players like Russian tanks than other nations tank.

For me , I'd go for all (yet it's a money drainer)

Heavy Tank for me T29 from US , KV-1S/IS/IS-3 for Russian Heavies , and the Tiger/P for Germany.

Light tank i would prefer the french as they (for me) have the advantage in their small size and really agile movements.

for TDs , I'd go for either the SU Series or the Stug/JgPz series (but don't go for RU TDs yet as i think they are gonna be a few tanks coming from the new patch)
SPG is a bit special , I'd go for French cause they have quite the advantage is speed so that they can reposition themselves when threat(s) come near them.
But don't take my case serious , everyone has their own likings to each tank.
So try out , ask opinions from other users and you'd be fine.
Ouh yea , add me up , IGN is in my sig. 

lol.... i always have these teammate, and it kind of frustrating during assault match
like the Railway devider Map, those heavies all camping on hill, not to defend the chock point, end up the attack team breach in

the other i really feel piss off.... i at the front, behind me are another medium, and 2 TDs, during encounter i am the 1st destroyed, and those at my back all fall back, even though the defending side already flash up on mini map, and those tanks are in 100% health. and they keep camping and shouting: arty, shot them.....
i really sweating how arty shot the target without anyone want to scout. 

during encounter, all camping at base duno doing what, some heroes even take cover at back of arty, when opponent cap, they so brilliant to ask atry: shot the side of the cap zone...... 

found there are a lot of the players do not care their team win or lose, they just dunwan to die, or getting themself some kills, that's all

like some games, we pushing from a side, and success to break thru that side's defend, but another flank just camping, end up we do not have support from sides... well they going to give nice name of 'sniping' instead of camping...
this is a tactic to flank opponent from their back. if whole team go to cap point, opponent going to attack u from 2 sides
lol... yesterday my team's Bison charge in front of the team  

that map can me difficult to breach for attacking side if defend properly, problem is too many thinking they are 'sniper' all hiding on hill, when base capture almost complete, only go down to the town..... imaging those KV tanks, with the low accuracy sniping on hill....

my experience on attack side, I was playing PZ38na, 1st i was scouting at the chock points, but seems my team getting some losses, and there are no defend at the cap zone, so i move to cap hiding back of a house, my teammate was go in to the town, that time defend side's KV 1 jz come down to took down my team mate, and another medium sniping at back of the rock. by the time that KV found me and wanted to kill me, the 1st shot hit the house, i've finish cap, and my team won, with me and another arty left, while defend still have 1 heavy and 2 mediums. i get 100% base cap that match, hehee