The worst tank you can play is the one that you have no fun playing.Period.
If you are trying to grind through a tank where you struggle to do an impact in your games,where you have no fun playing or even when you have a losing streak..that usually kills the fun for me..
This impression can be worsened by playing an underpowered (or flawed) tank,the tanks classed as scouts..or mostly any stock tanks...
If you want me to name specific tanks then,yes,the M3 "lee"..not because i think its a horrible tank..but more because of the grind you face if you want to unlock the vehicles it gives access to..
The 3001p because,quite frankly,its s*** when you get it. At the end,however,i had some fun with it (fully upgraded that is)
The Jagdpanzer-IV: because initially you cannot.mount.any.single.thing.before.upgrading.the.damn.tracks! (also the inital gun is quite useless..and even fully upgraded its not quite as good as other tanks & TDs on its tier).
Edit:
AMX 40: Tier 4 tank so you get to experience the full effect of the MM first hand..and even with a -potential- high speed..you will never achieve it because: that would involve you going uphill first(!) and good lord is this thing slow!
Like the first few tiers of french tanks the guns are a weak point to: the best gun you get acces to is a tier 3 gun (with ok damage,imo,but close to useless penetration). The armor,though,is -very- good..for its tier. Wont help you against a tier 6 or 7 tank,though. (and yes: you will face those sooner or later in this thing..)
AMX M4 (1945): Its big and tall,with nice straight (slightly sloped it has to be said) sides and not very good armor for a tier 7 tank: Upgraded the speed will be ok and the guns,although the same guns as the ARL 44,are ok too on this tier (because of the penetration,imo). The thing that kills the AMX M4 (and i know they are looking to improve this) is gun depression...In short: you dont have it..At all..Unless you are enganging another tank on completely(!) even ground..you will have to expose large part of your tank in order to take a shot at any vehicle opposing you.
Also a special mention goes to the stock T34 (US): And yes..i know that the thing is being relegated to a tier 8 premium tank as of next patch..but i just had to get this of my chest first..
Apart from the armor (same as the T29,a tier 7 vehicle) you have the 2 first guns (the stock 1 and the T5E1).Now. T5E1 is great on the T29,at tier 7. (even too good,compared to other tier 7 guns,imo)..but..at tier 9 it lacks penetration.And damage. It is better than the stock gun (tier 8 90 mm) but i think that is about the most positive thing to be said about it.
To get the 120 MM gun (and you want that.oh yes.you do.you -really- just want that damn gun.trust me) you have to first get the turret and then you can start grinding the gun. This means,in short,that you will be driving around in a fair amount of matches being (close) to completely useless.did i mention the amount of xp required to unlock the turret and 120 mm gun? Have fun doing that.