Hi,
I just wanted to share my build which I have been pretty successful with. Most of the games I end it with a siege damage way higher than everybody on both teams. And if the game extend past lvl 20, I often end up whithin the top 3 hero damage + xp contribution.

Here are some reasons why I built it this way :

Tier 1 : Survival instincts gives you a advantage early on. Using symbiote on a locust, you can do a lot of damage to "squishy" heroes that try to be agressive too early or are hiding in the bushes. Whether you solo your lane or have help, it really helps to support the attack early. If you have a Tyrael or Illidan in your lane for example, good luck to whoever is facing you they won't be able to do much.

Tier 2 : I'd rather have a reduced cooldown and more charges on nest than going global. The reason if that on most maps, with a good placement in the middle, you can mine pretty much every critical areas. I used to choose global range, but instead I changed the way I play to be more strategic.

Tier 3 : The slow from the nest is very very very helpful for your own survival and for you team chasing the enemy. Good nest placement can really help an ally chasing an enemy.

Tier 4 : Monstrosity. Needless to say that this is a beast to push on fortifications. If the game allows you to be agressive and place Abathur on the front lines while hiding, the combination of the locust quickly getting to the enemy creeps and your Monstrosity will bring down fairly quickly their fortifications. He is also surprisingly good at killing heroes when needed.

Tier 5 : The cleave damage will help go through those creeps wave like butter.

Tier 6 : These additional locusts will help do agressive pushes.

Tier 7 : Locust next with further more help to do agressive pushes.

Abathur is very sensitive to the team you have. If they understand you well, they will not waste the time you use symbiote on their heads. If all lanes are getting pushed hard against you, it will be harder to play agressively and place Abathur in a position where he can soak XP and push harder (locust have a time limit on their life, so the farther you are from the battlefield, the less efficient you become).

I found that the best way for a team to behave with an abathur on their side, is to group up, make a attack, kill 1 or 2 heroes, push while the other are either staying in their lanes because they are affraid, or while they are coming to help. Either way, you get momentum. They also have to stay as much as possible on the lanes you aren't for these attacks. You don't want unwanted attention on your lane, and be free to push.

Consequently, an enemy who understands how Abathur works, can make your life very difficult. A good placement between a fort and towers can take care of a Zerathul trying to kill you early. However, a Nova will most likely kill you if she comes on your side of the wall. She has time to do the necessary damage and stay alive. Rehgar can also be very annoying if you don't pay attention.

When your team has the upper hand, take the occasion to chose an agressive placement. When you have the feeling the other team is going for the boss, they probably are. Pick the farthest lane and push hard with your Monstrosity and make sure to be as close as you can from they fortification without being visible to them. Change lane often when you start being agressive. A good team, will notice your push and can guess with fairly easily where you are hiding. I know a skill like ballistospores can really help being more mobile, but you can find other ways. I find Prolific Dispersal way more helpful.