First, anyone who says the Sig P226 is junk has no clue what a true combat pistol is. Secondly, after the P226 9MM, it is hard to beat the GLOCK 17. Nothing else really need be considered.
The 1911 is obviously a proven combat pistol; however, with only 11 round maximum capacity, the only other viable options for double stack high capacity are the G21 or the HK MK23. Both are fine weapons, but the single most important objective for any combat weapon is shot placement. No round gives proven stopping power, speed/energy and shot recovery like the 9MM. To quote a Navy SEAL, "a lot of people say the 9MM isn't a lethal round, but tell me that after I've put two through your heart".
One poster submitted the G22 was the best choice, and while I do believe in affording everyone their opinion, mine is that the .40 is a bit snappy. Having owned and fired the 17, 19, 22, 23, 27, & 30, there is no question the best IMHO is without question the 17. All are fine weapons, and I do think any individual should go with whichever weapon they shoot best, but for a mass issue weapon I firmly believe the 9MM in a true full size pistol is the best possible choice. If you're going on price, accuracy, reliability you simply cannot beat the GLOCK 17 right out of the box. If you want a low profile no frills pistol that you can pull and shoot, the G17 just works. Fewer working parts than any other pistol, high capacity weapon in a proven round.
My personal favorite, however, is the Sig P226. I've owned the 220, 245, 229 and the 226, and of all the pistols I've shot, I'm most comfortable and accurate with the 226. My current cache includes the P226 9MM, G17, FNS 9 (really liking this pistol-try one), Colt 1911 .45, and a Kel-Tec .380 pocket "get off me" pistol. I'm lethal with all, but if I had to choose one, it would be the P226.
A newer pistol manufactured by FN is the FNS 9. This is a really sexy striker fired pistol made in the USA, comes with night sights, three 17 round mags, and has an adjustable back strap. Very accurate and just feels really good. It's about a half inch shorter than the G17, but has a full size grip. If you want to try a new pistol and have a lot of fun, I strongly encourage you to get one. Runs btwn $575-$625.
Simply put, for the M9 replacement, however, go with the Sig P226 or the GLOCK 17.