Let's see:
> Gmail
This one gained popularity when promising a whopping for free 1Gb was crazy good offer. People seriously thought it's a gimmick that won't last. Now that you can pretty much buy 1TB drives with pocket change it sounds funny, but back then it was spectacular. UX had very little to do with it, and frankly I still kinda avoid using gmail website, much preferring to use mail client.
> Youtube
Excellent idea, and excellent decision whoever decided to buy it. Social aspect is not that clear - youtube comments are notorious for being utter garbage, and while everybody is using youtube, nobody does social networking on youtube. UX is ok, not spectacular.
> Google Productivity Suite
Google drive and google photos are UX epic fail, IMHO. I'm still pissed about them killing Picasa, it wasn't excellent but at least useful.
> Orkut
Yeah, this is an undeniable success in social networking, and everybody I heard from in Google are equally surprised and have no idea what happened there and why it took off. I suspect it's one of those "right place, right time" moments.
> Chromecast
That works reasonably well, but does it have any UX to speak of? Then again, maybe it's exactly Google's forte - the products that don't need any UX beyond plugging them in and turning them on. That, however, I think would also kind of play to my point of technology being stronger then UX.
> They didn't try to become a "portal" like Yahoo
Yes, they avoided a dumb mistake, which is a plus for them, but I'm not sure that this qualifies as being strong at UX. Unless you say that there is to be no UX anymore beyond the very basics - which, again, works for some tasks, but far from all.