Its hard to say. Some of the games require money to actually thrive, with limits to bases or expansion unless you pay up.
By far the best browser game Ive played was Castlot. It had a great mix of things to do. City building, research, attacking, defending. A good clan system. Plus got to upgrade and equip your heros to win duals. The pay to win aspect was not too overboard. People could buy teleports, to swoop in on you, but it was hard for them to keep your towns. There are also events like world war every so often which were fun to join in on.
The downside is that the servers quickly emptied when a new world opened up.
Basically every week they would open a new one, and everyone would flee for the next one, leaving the place fairly empty. What killed it for me was when they decided to merge a bunch of servers together. I was ported into a server that had been going for 6 months longer, which had opps that were way too high level to defend off.
The game looks to be all but dead now. No new servers have been opened up lately, which is a shame, as it was a fairly balanced game.