The "easiest" race depends on your playing style.
Warriors tend to like the Orcs, because they are better at surviving actual fights, making it less of an obligation to wall off your civilians.
Humans have access to a lot of things, in a less cluttered way than the current dwarves, but need to be "above ground", so it means a lot of building, rather than digging rooms, as well as quarry digging.
They are pretty weak, so walling off is nearly a necessity.
Dwarves play mostly like the vanilla, with a ton of added contents that clutter the various menus.
Still, it's things to do in addition to the vanilla, and none of it is required to survive.
The other races are a bit harder to play (in my experience), because they play really differently, and requires that you know your basics really well (gnomes and "power", for example), or introduce completely new mechanics (the skeletons/pilons).
The humans have the added benefit of offering a simple way to show off some of the other races' specialities (through trading and workshops), so if your basics are OK and you are not afraid of building above ground, it's a good way to discover the pack and the races.
For Adventure mode, it does have a lot of things, but it's completely all over the place, because MDF was never intended for this mode.
It has new slabs, or towns for other races, but you will often be frustrated of how it feels, compared to vanilla adventure mode, that is somewhat consistent.