Best soldier that can be created by 40k

[Manus Celer Dei]

Well, the "Better, but not that much better" bit in Blood Games was comparing the Custodes to the Imperial Fists, so maybe the Word Bearers and Thousands Sons just suck

[Black Admiral]

Well, the Word Bearers are descended from one of the less capable Primarchs when it comes to combat prowess (Corax and Lorgar have it out in TFH, and the fight's basically Lorgar trying desperately to stay alive, while the Raven Lord's fighting to kill).

As for the Thousand Sons, well, given the presence of Valdor at Prospero these presumably weren't just the generic mook-Custodes with armour & Guardian Spears, which is presumably the point of comparison in Blood Games (now, if I could recall where my copy of Tales of Heresy is...).

[Korto]

Wouldn't best soldier depend on the mission? You can't really have a soldier who's the best at everything. Everyone has their specialty.

OK, to try and put some definition into it (sorry if this counts as a de-rail)
You are employed to hire some troops. You have a "reasonable" sum of money, to hire one troop type, with their basic support infrastructure, and can choose from the entire 40k galaxy. Thing is, your emplyer wont tell you what their task is.

Who do you choose?

[xthetenth]

For mercenary work, I'd be sorely tempted to go with a balanced Tau hunter cadre. Most of their weaknesses aren't in the kind of thing mercenaries would be expected to do (taking and holding, getting in drawn out slugfests, and so on), so their mobility overspecialization shouldn't prove too harmful. It's either them or the IG though. I shudder at the price of the Necrons or Dark Eldar, the Eldar are fragile and liable to overcharge hideously, even if they are tempting as a tau replacement, guardians aren't professionals and in general they're very short ranged and that makes me leery. Kroot can't offer heavy kit, Chaos is usually too tactically incoherent (although I haven't seen too much tzeentchi in action, I bet they'd fare better). That pretty much leaves the IoM and Tau. For a fast single strike, I'd be very tempted to go with the Tau, they're optimized for winning battles, while for much of anything else, I'd go with Guard, which is optimized for winning wars.

[The Yosemite Bear]

how about Lillith Hesprix (sp) sure she's a slave, and an arena fighter, but she's immortal, and has vibroblades in her freaking hair?