This is an incredibly "open to interpretation" question. Everyone will give you a different response but it really all depends on what mods you plan on putting on your car and what power figures you're looking to achieve.
You're talking about a strong motor that has a hard pull and good reliability, but it's not at all the motor itself that governs those things in our cars. Sure a 2.5 liter block will produce slightly more power than a 2.0 liter block... but it's your turbo and supporting mods that will define how much power your block has, when the turbo kicks in, and how reliable it is... those things depend on supporting mods... and MOST importantly a quality tune.
Also, when you say "building a motor" are you talking about building internals, or assembling a long block?
To have a high revving block you will not only need a built short block but also head work done to support the higher revs. Some blocks/heads rev higher from the factory, I believe it's certain JDM ones but I do not know those details.
As far as BEST, "if money is not a limit" hypothetical situation... you asked for it, here it is:
IAG / Willall Stage X Billet Aluminum EJ25 Subaru Short Block
Billet means that the block itself is not cast, but is machined from billet aluminum. It's capable of holding over 1000hp.
If money isn't an option, that's the "best".... for $15k without accessories. Please note, you will still need built heads, a turbo that's big enough so it makes sense with that kind of motor... completely different fuel system and just about everything else on the car including built transmission to hold the power without exploding and a clutch that can transfer that power... and once again... just about everything else.
-----------------------More reasonable approach
Pick your maximum reasonable desired horsepower (in HP, not WHP) for this particular car's FUTURE plans... add 20-30% to that figure for headroom in reliability (I just made that number up)... and buy an IAG built short block to reflect that figure. Find built heads or quality cams/springs which will allow you to Rev higher.