10
Personally I like the Sentinel in 2/3 the best but in 1 they don't really have the specific niche purpose that they seem to in the sequels, since all dual classes just get half of the powers from each.
ME1 I just go full Engineer and select AR training.
It's always AR training.
I have occasionally moved to adjacent classes but I pick whether or not Shepard is biotic in the beginning of the run and don't deviate into it if I don't originally have it. So for example I did Infiltrator in ME1 and then switched to Engineer in 2 and 3.
Sentinel is really cool because you feel like you're able to handle anything on your own. You have overload for shields, warp for Armor/Barriers, and throw and cryo as utilities for crowd control. To top it all off, an additional layer of protection that detonates to stun surrounding enemies.
Sentinel plays like a Command and Control unit, you lean heavy into squad combos (Samara and Thane is a good squad build in ME2, lot of combo and anti defensive power options) while playing to the strength of being able to constantly harass everyone around you single handedly.
No one is ever safe with a creative Sentinel on the loose.