I have several guns that have been 100% reliable:
-S&W 1911SC (very accurate)
-SIG P250C (though not all that many rounds yet, very accurate)
-SIG P290RS (crazy accurate for the size)
-Ruger LCP
I had (but no longer have) a few others:
-SIG P229
-Ruger P89 (though it was very inaccurate)
-I don't remember my CZ 40 ever having a hiccup
I have (or have had) two others that weren't quite 100%, but considering the round count have been quite reliable:
-Charles Daly 1911 (over 10K rounds, not many hiccups). I unfortunately sold it, still have the next...
-CZ 75B (well over 10K rounds, I can probably count the number of hiccups on one hand)
I'm sure I've left something out, and (due to the criteria) I only included those with any hiccups if they had over 10K rounds and few hiccups
So...best as in 100% reliable (or may as well be considering the ratio of rounds and hiccups), great accuracy, comfort/ergonomics, and how much I simply like it... It would be a toss up between the CZ 75B, S&W 1911SC, and SIG P229.
Edit:
I need to partially modify my answer. The question doesn't say best semi, but best "out of the box" semi. It has been a long time so I almost forgot, but my Charles Daly was not initially as reliable as it later was. The problem was the mags so it was a cheap and easy fix. However, reliable "out of the box" implies no modifications or changes whatsoever and having to buy aftermarket mags (I usually used Shooting Star and Wilson) is not quite "out of the box" (though not quite what I'd define as a modification). Just in the name of full disclosure I figured I should add this information.
Oh, and if you count regular routine maintenance as no longer counting as no modifications and repairs, around the 5K round mark I replaced the mainspring on both my Charles Daly and my CZ75B just in case.