This is really difficult to answer.
The Profile Stats are something I regularily use, but could easily do without. I don’t have Windows® or GSAK, but I’ve already started writing statistics code myself (in Korn Shell out of all programming languages), and the best thing about that is that it uses a local list of found caches, in which I put those of all platforms, not just GS (but also OC, TC, and others).
The TB discovery tool is useful after attending an event, but also something I’d probably write myself using screen scraping, or maybe even the API, though it would put some pressure on me to finally learn how to use OAuth ;)
The maps compare feature is dead useful, but I only need it a couple of times a year at best. I don’t think I could do that myself. (I am a programmer, have been for 27 years, but my focus lies elsewhere.)
The Virtual GPS is something I really like, as it works much better than what GS has, especially as PGC’s site performance is much better than GS’ either, but I could probably do without.
Not exactly a “thing”, but I like how PGC makes discovering new goals easier – whether it’s “am I eligible to log this-and-that challenge already? (checker)” or “hm, I wanna cache, but don’t know where, let’s look at the tools (hidden months, D/T matrix, challenge finder) then decide to work towards one of them”.
And, finally: while I would be capable of implementing my own statistics, and have my own statbar already, PGC is a great source to demonstrate what’s possible (although (while I don’t know, as I’ve never ever seen GSAK, I suspect) most is probably inspired by other tools), i.e. a fount of ideas.
I’ve become PGC PM relatively quickly, while it took over five years for me to do that to GC.COM – and, recently, I’ve even donated a bit, to show my appreciation.