Worthwhile - Volcarona. Larvesta is weak and frail, and it takes a long time to train, but when you get Volcarona, it is really strong. With Quiver Dance, it can sweep just about anything. It is inarguably the best Bug type in the game.
Easy - Sandile was easy for me to train. It's attack is good, and it learns good moves. Plus, I got a Naughty one, and that certainly helped. Moxie also helps it. Another would be Darmanitan. My Darumaka was great even before it evolved, and it played a valuable role in defeating Ghetsis.
Hard - Tynamo. While Eelektross is worth it, it is hard to raise a Tynamo early in the game, with it's bad stats and moveset. Gothitelle also comes to mind. I used one on my most recent playthrough, and it is just too frail, it's HP is too low for it's defenses to actually work, and it's special attack is decent at best. Even with a Sassy nature, which would increase it's special-walling skills, it's speed is rubbish, it's HP is rubbish, and it's physical attack is certainly rubbish.