Since Skarner was f2p I tried him and used different kind of builds with him. The most effective build is this one, especially if you take the top lane. I tried him some times at ranked games and welly I was very strong

I pick AP p lvl Marks/Seals/Glyphs, because with lvl 5-6 (when he can actually beat nearly every champ in 1on1) he has some nice Bonus on his skills. As Quinteresses I pick AP runes so he won´t be too weak in early game.

I get defensive masteries so I can win 1on1 fights easy and I also get additional AP and magic penetration because the most damage I deal is magic damage and I don´t want to get Soccers Shoes because Ionian boots give me a better bonus.

First I get Sheen so that I can do massive damage in melee combats, and I go solo top so this kind of fights happen very often. Also because im very offensive with skarner I get ganked early, but when I have Sheen and my Spells I can make double kills and getting away with 100-250 hp. After that I get my first Hextech Gunblade which increases my damage/AP and gives me some nice ls/sv. Normally Skarner is already very tanky at this moment, but because of end game champs which could kill you at about the 40 minute, I get WA and GA which also helps me to survive when I get ambushed or focused in team fights. After that I get my second gunblade so that I deal way more damage and get more hp back. In the end I get Lich Blane so I deal actually at 5 of 6 attacks additional 350 bonus magic damage and, thats why I get the Ionian Boots ánd the blue buff as often as possible.

First I thought that it would be better to skill the E skill first because it gives me a nice heal when and enemy creep wave comes or in 10n1 fights, but because of its long cd and because you dont have a 100% chance to hit your enemy I prefer to skill the Q skill first.

Because Skarner is very strong in 1on1/1on2 fights, and ganks good I pick exhaust/ignite with him, so that it is guaranteed that you get the enemy (especially cause you also have your ulti which prevents champs like akali to get out of attack range)

Pros:

-High dps damage
-Hard to get killed
-Nice cc because of his ulti
-Ms bonus and slow make him good for ganks
-Can engage teamfights without dying and making nice AoE damage.
-Can dive easily because the enemies can´t get too far while being slowed

Cons:

-Short range abilities
-Easy to get killed early game when getting ganked
-Can´t "rule" the whole match, he needs his teammates to make his cc even more effective

In teamfights it´s best to pick the enemy carry with your ulti and drag him into your team so you can win the team gihts easy. If your ulti is on cd you should still go to the enemy carry and prevent them from fleeing or attacking your team while slowing and harassing him.

If you get Sheen it will be very easy for you to farm, because your Q skill has a low cd and low mana cost so you can always use it to make your attacks stronger.