I'd work on your core (
DP's, and Oblits) and decide which combination of troops choices you like.
Then I'd build it and give it a whirl.
There is no 'perfect' army out there, there's just the
FOTM army (right now, that's Orks) and in a month it'll be Demons (for a month or two). Then it'll be smurfs, fanged smurfs, then rippy eldar.
Anyway, pick the troops you like and go with those. You can easily just sub in some troops and see how you like each flavor.
I personally favor Thousand Sons and Emperor's Children, because I can deal with every type of balanced army that exists. There is nothing wrong with
PM, I just don't have a 'get pounded on' army style. I think Khorne Berserkers suck currently in 4th edition; but they'll be a bit better when they can start in a transport, be deployed 12", go 12", be deployed out 2", and run
D6". If you go first, anyway; which works differently in 5th edition so it's a bit easier to get away with it.
Most times they get shot to pieces, but a bull rush of zerkers straight in your face does require you to play a very smart game in the one turn you have to kill them all or the impact will roll your army. Of course, if you go first and pop all the vehicles...it's not very hard to win.