I'm not so used to boss fights and getting tek every time I've played. My favourite ones are the most vanilla ones in The Island map, but I think the ones that unlocks the most tekgrams are Extintion and Genesis 1. They are also very good maps to farm element.
To follow the Extintion pathway to unlock tekgrams, people tent to recommend to start defeating the desert titan. If you have a lightning wyvern you can farm it pretty easily. It is the eassiest 'boss' fight for early Tek, then, you should keep killing the other titans to unlock the rest of the engrams on the map.
The map is full of ratholes, so if you are playing in a PvP server you could follow this process hidden from the rest of players, with your dinos in cryopods, an item introduced in this exact map.
Here you have a link to the tekgrams per bossfight.