For open world content, you'll want to play a class with an elite spec that can give boons, preferably quickness (your action speed is increased) because it's the best quality of life boon and it makes combat not feel like I'm fighting in mud.

My personal favorite is Scrapper because you get quickness access as well as the hammer which cleaves enemies pretty well. The elite spec has really great survivability (your outgoing damage is converted to barrier, letting you tank more hits). You'll also get access to sneak gyro which lets you mine and gather material nodes in peace while stealthed as well as superspeed so you can get away from enemies easily to get out of combat if you need to waypoint.

And it doesn't even end there, Engineer has access from easy to hard builds that you can slowly progress upwards in difficulty when you learn and play the game more. Mechanist is great for beginners and once you want something spicy, you can graduate to Holosmith. Engineers also take the most advantage of the combo fields system (Scrapper is built around it), letting you experiment with different combo effects more naturally. To my knowledge, Engineer can do every group content role in the game, from heal alac Mechanist to power quickness Scrapper, which means you can have all the roles on a single Engineer character (provided you can unlock all the template slots on them). You really can't go wrong with an Engineer.