Sentinel for me - I love having access to biotics and tech. Sentinel is a well-rounded class that can work well with any teammates. It's a fun class in all 3 games, but in 3 it's amazing - 90% damage resistance with fortification (soldier ability) completes the class with all 3 types of moves, access to all weapon types, and 60% increased power damage and force from armor abilities alone. Can't wait to see how Andromeda's new system works, would be awesome to have a sentinel with more access to soldier abilities as well. All the classes are really fun in 3 and I could play any of them and have a good time. :)
Adept for sure. I know that isn't a very popular choice, but for me Adept is the best classe ever because even Mass Effect 2 insanity mod is easy with adept's abilities. If you pick energy drain as bonus ability, you're probably invincible. Adepts can destroy the enemy's armor and control the field in seconds and can throw away everyone who tries to use melee attacks. I know adept isn't the most resistant class, but if you're able to control its abilities with wisdom, adept's low health isn't a big problem. And sometimes is hilarious to see your enemies flying and screaming (Ikr?) if you use pull.
Soldier, hands-down. Ya got biotics and techies and so on in regards to your teammates, so being a well-rounded weapons expert that can speed up reaction time and use an entire arsenal of lethal arms seems to even things out a bit, as far as squad selection..
I enjoy soldier the most because they are masters of guns, and the abilities match well as fillers when you have to reload or take cover. Sentinels are a close second as they can be super overpowered if you build them right but I still prefer soldiers.
I like Engineer the best in all 3 games, but for different reasons in each game. In ME1 they have the fastest cooldown times for their primary powers, and leveling many of the powers actually improves multiple tech powers not just the one you are leveling. For example, leveling AI Hacking improves the cooldown times for Sabotage, Overload, and Damping. The one-two-punch of Sabotage and Overload will pretty much render any enemies as helpless as babies. It also means Shepard handles all the hacking, so you have more freedom with your squad choices instead of being forced to use Garrus or Tali so you can get into locked things. In ME2, there is one reason alone I like the Engineer best....scanning. I freakin HATE scanning in ME2. It's the only game play mechanic in the series that I hate. Engineers get a 25% discount on resource cost when upgrading in the tech lab. That means much less scanning required, and that's more than enough for me to call them my fav class in ME2 lol. In ME3 I love the improved(over ME2) Combat Drone and the Sentry Turret. Once upgraded with rocket launchers they do more damage/kill more enemies than my squadmates lol.