The third generation G17 "should" be the strongest since it has a locking block pin added (added for the larger calibers)....however, since the G17 was originally designed around the 9mm without the locking block pin and the fact that there are many generation one and many more generation two pistols out there that have fired mega thousands of rounds, it really doesn't matter....any of them are up to the task. I "think" that the locking block pin was added somewhere during the generation two production for the .40 caliber pistols and became standard for all the like frame sizes thus incorporating the 9mm's as well.
It will just depend on whether or not you want the finger grooves that the third generation has...and if not, then just search for a used second generation and drive on...or if your real lucky, you might find a first generation G17 with the pebble type grip finish...they're gettin' kinda scarce these days.....
If buying used....just be aware that some of the E series serial numbered pistols had an issue with some weak frame rails but the frames are replaced free by Glock. There is info on this listed somewhere on GlockTalk. Other than that, the G17 is a very good choice.
