There is no "best build". It all depends on the match up, enemy build order, map, etc.
This is 100% truth
All of his wargear is viable and may vary on the situation, though there are a few builds that for me are the most common to use:
One was mentitioned by riku
Another is power sword (into fist late t2) alacrity - halo (into stendard late T3) This is a very energy intensive build, I often used it against eldar and orks as they won't be able to kite the fc with the alacrity and they will have problems in dealing with the iron halo even if it's not used at it's 100%, although power sword is not needed in most cases, get it if you want to play heavily on the FC, other times you might not want or need the halo at all, the ps + alacrity will do enough work.
Then you have the quite standard fist or thunderhammer - artificier - teleport pack it's quite self explainatory one provides AV + stun on demand on vehicles and the other is great vs blobs, the teleporter also makes you take 0 damage on the way or an escape mechanism.
Another common build is the super tanky build Any weapon + artificier + halo, best used vs slow races like chaos/gk/sm mirrors, especially when your opponent gets ASM, it also may work vs IG, as the iron halo gives them quite some troubles.
this are imo the most 3 common builds you'll see, but it doesn't mean you restricted to those, in more or less specific situations you may want any combination of the wargears at your disposal, under this aspect the FC is a very versatile hero.
And yeah, I forgot to mentition, you don't need to always heavily invest in your FC, alot of the times your standard FC will do just fine in T1, sometimes you may just want the PS (power sword) or just the halo the annoy your opponent and buy you time, but you'll get a grip on when to make those purchases when you'll get some experience, for now, don't be afraid to lose and experiment all your options.
That said, as riku suggested starting scout > tacts > scout > t1.5 is the standard opening in 1v1, in team games starting scout > tacts > gens > t1.5 unit is usually enough, but more or less all builds are more or less viable, as always depending the race and your opponent's build/hero.
Other more uncommon builds are Starting scouts > 2 scouts > tacts or double tacts or +2 scouts or 2 scouts > 2 ASM.
