Without getting into specific brands/models, the easiest type to pick up with ZERO practice and sound good is a box call or push-call. But with most box calls, it limits you to realistic yelping only without some added skill.
The easiest to use well (sound good), with just a LITTLE bit of practice, and my favorite type, is a pan/striker call. With 1-5 minutes of practice, yelping is easy. With another 5 or 10 minutes of practice, purrs are pretty easy. With another half hour of practice, fairly realistic soft clucks can be made on the edges of the pan. I always carry 2 pan calls: a slate pan/wood striker for normal conditions, and a glass pan/acrylic striker for rainy conditions, or just to change up the sound a bit.
Mouth calls are difficult to make sound realistic without a lot of practice. In the early season, and/or where there's not much hunting pressure, and/or if there's a high(er) tom to hen ratio, then a not-so-good mouth call yelp will still bring them in usually.
On locator calls, I don't mess with owl hoots or crow calls anymore, because I found that a gobble call works best as a locator, or for that matter a loud hen yelp. I use one of those black rubbery gobble calls that you hold in one hand and shake vigorously for the gobble. Some people will say that that is dangerous, to make a gobble sound, but if someone is out there stupid enough to shoot a dude dressed in camo, carrying a shotgun, and holding a black rubber doodad in his hand, then all bets are off on that kind of yahoo anyway, so I don't think actually gobbling is going to matter.