Skeletal build
Yeah so, I'm sure you've noticed that there's a TON of SP left still. I'mma just go over the options you have.
Wisp/Stalker: Choose one of the lil guys and max it. Wisps do less damage than Stalkers, are less aggressive and cannot OTG, but they're arguably easier to summon (level 1 florae shock is all you need). Multi-summoning Stalkers requires a pet, but they hit like trucks.
The High spirits (Glaerin, Flame Hulk, Aquarius, Dead Burger): You can max all of 'em or two of 'em. It's your choice, really. Regardless of what you end up going with, max
Glarine Glaerin as it has the highest damage output among high spirits. Flame Hulk/Aquarius are good second choices.
Aukuso/Freet/Sandor: Your completely optional contract summons. Freet's a flying pocket launcher, what with his flamethrower, fireballs and general speediness. Sandor whacks stuff around with his sword and has an extremely useful defense aura that tanks damage for both you and your summons. You can get EX Sandor without maxing the original; it only has to be level 5.
Summoner's Train: Pretty neato when maxed, especially if combined with Sandor. Not necessary though, and is more often used as a "ERRYONE OVER HERE" kind of skill.
Zealous Creature: Not gonna lie, I love this skill. A lot. Beefs up your summons for its duration. Only one point is necessary since it gives you the biggest bang. You can max it if you've got the SP though.
Elemental burn: If you can really, really, REALLY be bothered to keep all of the elemental marks up, then burn does increases your damage output. Me? I'm lazy.
Anywho, here are both of my summoner's builds.
Wisps + Contracts - Much easier to play and still fairly good as far as damage goes.
Stalkers + Spirits - Vastly superior DPS, more difficult to use properly.