New Reward Tank, Berlin PvE Mode and Berlin Map | World of Tanks Road To Berlin, IS-2 Shielded

You can get bonus x15 tokens by completing Berlin missions in normal games (must be tier 7 or higher and must select the specific tank you are completing on) from mission tab. 

Without xp bonus on Berlin missions your progress is slower than a snail (+21 xp for victory with no bonus - 315 with bonus for example).

5 tanks (1 TD, 2 Heavies, 2 Mediums)

The Russian Heavy and Medium are very strong
The American Heavy and Medium are very weak
The Russian TD is good (no other TD to compare it too) It suffers the TD issues and benefits from the same... very hard to remain "concealed" in a TD on any Berlin map from AI)

Hands down the single strongest tank is the Russian Heavy "Ironside" line. 

Each of the 5 tanks gets a free x15 bonus the first time played. Each of the 5 tanks get a free x5 bonus each day. You can pay to win with x10 bonuses till your hearts content or you are as poor as the Russian soldiers fighting in this depicted conflict. Choice is yours. It is a waste of time to play without tokens, better to play normal matches and get the missions for the x15 token currency. 

You do NOT have to finish all 5 stages of the campaign to get the IS-2S, only need to compelete stage 4.