Aside from the Locust Builds which are kinda just there for those who want to fool around with Locusts, these are the most competitive combos I have been able to use.

Support, Damage and Push: This uses the basic build of support and damage from the pre-rework. You want to use it on mostly on a tank or very effective damage dealer at the start. Once you get stab at level four you can combine your heals and shielding with more damage, and more importantly, range to finish kills and apply pressure to hovering enemies. Don't forget to check bushes for your host. At level 7 you get networked carapace which lets you heal two or more allies at once. It will also let you shield waves of minions and mercs to help them push. This is a pretty huge impact on xp with your shield being able to ignore minion damage and reduce tower shots. Enemy teams can less frequently wipe waves with area attacks if you time it properly.

Monstrosity is your ultimate choice in nearly every scenario now. You can use it to push, help grab camps, or bully lanes, or join team fights. With your rapid shields and ranged attacks, this beast can poke very well and punish those who enter melee. Tunnel back home after a while to focus on your team for a bit. Try and keep it alive when you can. At level 13 is another charge of your stab. This is important for your Monstrosity as well as zoning away enemies and finishing kills. Keep the pressure up. At level 16 is another shield talent that lets you boost speed for 40% for 3.5 seconds, which is half the time the shield is up. Because you have Network, this because a tool to ensure escape or prevent it for your team or opponents'. Then at level 20, you can choose between Monstrosity and Twin Symbiote. In this build you grab Monstrosity for the double melee damage. But if you're having trouble building it up or your team surviving, then Twin Symbiote may be a better choice. By getting Twin Symbiote you can extend your Networked Carapace's reach. It also tells you exactly when you can hit more than one teammate when you see the little symbiote twin pop up.

P.S. If your team comp doesn't need as much healing from you, you can swap out Reinforced Carapace for Pressurized Glands at level one for extra damage and pushing strength.

Melee Monstrosity, Spine Push: This build chooses a few different talents to extend the range of Spine Burst, and increase the heal to Symbiote host vs enemy heroes. Your Monstrosity gets the most benefit from that. Most importantly is the pushing power that comes from combining an extended Spine Burst range to hit all minions, with a Networked Carapace. It can also add poking power and ability to finish kills even when blocked by minions or walls or normally out of reach. Your Monstrosity will also be adding damage with it's aura to add to it's potency in wave clearing and damage on multiple heroes.

Twin Symbiote Builds: These use Twin Symbiote to double the stabs and spine bursts, extend reach for carapace network, increase the amount of speed boosts, and give your Monstrosity even more capability to act as a mobile support and damage dealer even when near death or out of range attaching a twin symbiote to a teammate. Makes for a very strong late game ability that suddenly doubles your effectiveness.

Auto Attack Support: This enhances assassins mostly to turn their quick and reliable auto attacks into more effective weapons. The speed boost, with additional healing from spine bursting heroes, and a twin symbiote to make sure when they stay back that you can still heal and reach out to stab at targets are all useful tools. Be sure to communicate to them to always try to stay close enough to double up on symbiotes so teammates can also get auto attack boosts and shields. Can still be used on melee hosts.

Tank Cloning: This build uses clones of tanks and bruisers to provide them with shielding and a bit of extra ranged weaponry and healing when cloning is on cooldown. When it is up, as long as it's up, the shield benefit is quite strong and refreshes every 5 seconds. Since tanks have high health and crowd control to defend themselves, it makes the shield that much more effective on them then others. Your basic crowd control from your own clone can also contribute to this better as a tank from a lack of talents or heroics that make damage dealers so good.

Locust and Mine Pushing Focus: This is more for fun sakes, but if you want to try it out go ahead. Use the increased mine production and Network Carapace to push in early levels and then enhance that with Locust spawning in the later game. Clone your teammates as a form of evasion if you need a quick escape to help confuse the enemy of your position. Be sure to use Shift+Click a lot and place plenty of mines for vision.

Tunneling
Start of match, switch lanes for mine range, if walls are threatened, grab objectives with mines but dont get seen, follow up with hearth, get a slapathur kill, and escape or taunt enemies.

Locust
How short the duration is, the dangers of death and time spent babysitting body, the slime map to your body, the goal of uptime, and the lack of effective team fight talents.

Symbiote
Stab, Spine Burst, Adrenaline and Carapace.
Animation time, cooldowns, durations, canceling, map awareness, holding fire, obstacles, hovering, range, time for mines, constant uptime, finishing kills, preventing deaths, speed boosting, turning the tide, twin symbiote, and network carapace.

Toxic Nest
Not for damage usually so much as spotting enemy and dismounting them, making them paranoid, damaging waves, mercs and bosses, drawing fire, finishing kills, zoning and a tunneling tool.

Monstrosity
Act as mobile platform for symbiote, heavy melee damage, expendable, global response, escape, draw attention, push lanes, avoids heroic targeting, succeptable to non-hero targeting, needs feeding time, will be abandoned by team, slow base speed, and cannot use adrenal overload.

Ultimate Evolution
Fight over team objectives, hold back enemy, long cooldown, two lane maps, lack of symbiote, tank shield, lack of talents.