[Keeper of Dol Guldur]

Sauron's eye, no doubt, was fixed on Rivendell because the Nazgul found the ring at Weathertop, and lost it at Bruinen. While the Nazgul realized that the gushing torrent of stamping water-stallions that mangled them must have come from the elves, and Rivendell was supposedly in that vicinity. Lothlorien was a much more powerful place. You asked which kingdom was greater? Easily Lothlorien. For starters, Imladris wasn't even a kingdom, it was the Last Homely House of Elrond Peredhil. It was his home, and a household, while Lothlorien was the golden core to a vast forest. Rivendell was never referred to as the 'Heart of Elvendom'. Aragorn who grew up there, was obviously a good judge as to the power level. Not to mention, Elrond was a Lord, but Celeborn and Galadriel ruled over a people, the Galadhrim, while Rivendell merely acted as home and haven for local western elves. It's pretty clearly stated that Lothlorien is the last great kingdom of the High Elves (of course, with many of their Sindarin and Sylvan brethren). After Lothlorien, I'd say comes the Grey Havens, and then the realm of Thranduil on the level of power. Rivendell had something the elf-realms didn't have though, lots of Noldorin elves. Elrond himself was important, and his children, but the likes of Gildor Inglorien and Glorfindel, and Erestor, are not to be taken lightly compared to a Legolas or a Haldir, who in their own realms were considered quite powerful. I think the realm in Northern Mirkwood had the largest population of elves, but they were an entirely rustic folk. Lothlorien (which had Celeborn, Galadriel, Nenya, and an entire city, not to mention scout flets out as far as the edge of the enormous forest, and more locations) was easily the strongest.

I just noticed that every elven place seems to have its own river. Common sense to build near a river, I guess, but the Bruinen, Forest River, and Celebrant (with Nimrodel) are pretty special.