the most conservative is the lcms probably because they are the German branch, the scandinavian synods were founded by Pietists who tended to be more arminian in theology. after world war 2 , the German population significantly declined which is why there are more Lutherans of the ELCA, Lutheran Brethren, etc than LCMS; but German has become the main language again so even Scandinavian have to learn it to read the big theologians anyway. They're all "Germanic" lanaguages except for Finnish; but even Icelandic is related to Norwegian. even Finns write their stuff in German too; just trying to understand the Finnish theosis debate requires reading tons of German books which isn't possible for me so I'll just tip my hat to those who can. It seems like German has become the de facto liturgical language if you want to study anything from 1800s or later, once you get past the Lutheran orthodox period where they wrote a lot in Latin still; the most conservatives are still the Germans.
WELS are also German, Too.
The Wisconsin Evangelical Synod.
There are free Lutheran churches, but I'm not sure if they are more conservative than WELS, LCMS or ELS.