What tank lines you regretted grinding?

Sael_:  Yeah, every single Japanese tank I got, I found very underwhelming... It doesn't seem suited to my behavior on the battlefield, which is a shame, as I enjoy hearing the crew speaking Japanese in my ear. 

Sael_:  This heroic-tier answer makes me want to put forward additional questions:1. What lines are the best to grind when you're new to the game?2. What lines took you the longest to learn? Were there any that you thought, you shouldn't have ground so soon, before nailing other important aspects of the game?3. Which lines instead you ground and enjoyed, despite everyone telling you those were vehicles too hard &/or under-powered to play? I'm asking because I'm curious to learn if there have been lines that you hated playing in the beginning but got better when you became more experienced players, and if you have any advice for newcomers basing on how your WoT life has been in this regard. Plus, even among CMs, we have heated debates over what to do when you start - some of us believe you should play the easiest vehicles and get a feel for the battlefield first, others advise instead to look for the class that suits your play-style the best and move on from that. What do you guys think?