I always chose to play as a soldier in the original trilogy on 360 and PS3 and now that I downloaded the Legendary Edition, I chose to play Mass Effect 1 to have my Shepard a Sentinel.

Sentinel is really good in the first game (all games really) but between these 2 i would say Soldier because you can use heavier armor that looks cooler, also never played as Engineer so i can really speak on It.

Between Sentinel and Vanguard, which one is the strongest?

For the first game i would say sentinel, considering we face mostly Geth, also having decryption and eletronics is great, Vanguard got really good in ME2 and 3 imo, but it was still good in the first, Vanguard has Adrenaline Burst that let's you immediately cooldown your powers so it's not that far behind, but i still would say Sentinel because of the versatility.

True. Thanks for the information about both Sentinel and Vanguard