skarner or hecarim whos better

They both have their defining qualities.

Hecarim I never played personally, but he has more chase and rushdown potential. His early ganks are strong, however he is easily counter jungled and usually has to not take flash for ghost inorder to catch people early game.

Skarner is late game destruction, he is very bulky, hard to build against (his abilities do AP but his autos are AD), has hard CC allowing him to stick to people, and a shield that once again defines his bulky abilities. He's an amazing 1v1er while in his crystals.

The downside of skarner is inorder to get his ult off, you may get a lot of enemies and teammates laughing at you for flashing for it... literally that's his combo: Flash - > R - > W - > Q - > turn around (face them the other way around to get behind) - > E - > AA. If you mess up this combo, say bye to your flash. His Q is also very spammy.. if you played cassiopeia and used her E you'd get it... basically you're mashing your Q until it breaks. However skarner is still very fun, his speed in crystals is hilariously fast, he can carry very hard late game, and he is a very simple champ to get used to.

It really depends on who's playstyle you prefer, they're both different.