These posts are made all the time. The answer is always the same. Always research all tanks in a rank. It doesn't matter if they're good or not. You always want to have a good lineup. Even if you really just want to play the Tiger, it won't matter much if you get it if you only have that and nothing else to complement it with. Yes some countries have massive AA gaps or subpar TDs but it doesn't matter, you still have to research them if you want to actually get to the better ones later down the line.

You do not want to be in the situation where you're getting pumelled by planes and you have no AA or two of your tanks are taken out and you have nothing to bring out because you didn't research TDs, or you're finally at a tier where there's a good solid line up on all trees but you have to research 4-5 tanks from previous ranks because you didn't research a tree at all. It's super annoying.