As said, avoid tribes like the plague (of Justinian

) - they make some nations nigh on impossible at the start...
I would still recommend Bactria (I mean I recommended Georgia as a started nation on CKII so my choices are...the roads less travelled...) but I would possibly add Kush (modern day Sudan):
Large economy
Good population
Their geography is one river - you can block it with a fort or two and just defend
People to the east are weak (Somalia/Ethiopia and Arabia)
Probably slightly harder than Bactria, but Egypt are easier to deal with than the Maurya.
Another option would be Capaddocia:
Protected by Antigonids, buuut an event means you get put under Armenia and thus not steamrolled by other diadochi.
Decent economy
Decent army
‘Major’ nations to avoid if new-ish:
Epirus - missions haven’t been changed and so are nearly impossible, also they have nowhere near enough manpower to achieve objectives (mission 1 - attack macedon...)
Athens/Sparta/any small greek city - a Diadochi or defensive pact will end you
Etruria - much harder since Rome got a +2.5% levy buff...
Syracuse - if you can take Sicily very quickly...but their bonuses are for mercenaries which you cannot afford early on...
Antigonids - if you can survive first war, you will be ok (hint - attack macedon as soon as you get claims, take Corinth and end the war. The other diadochi never pile on you and make it much easier as they never ally with each other)
Seleucids - too much culture mashing and many rebellions if not careful
Massalia - good god no, no
In summary - Bactria or Rome, if you want a diadochi try macedon
Good luck!!