I go to the Darkmoon Garden(AKA Moonpit) for fights of honor.
I go to the burg to fight ninja-flippers.
I go to the forest to be a ruthless animal that kills gankers(if you ever go to the forest, expect gankers. It is why I go there. You'd be surprised at how many people actually dislike people that do honor fights there).
I go to the lord-soul areas for random invasions.
I go to Sen's Fortress for incredibly easy random invasions.
I go to the Township for standard dueling(even though a good percent of them are as ruthless as the forest).
And I go to Anor Londo for killing groups of questers.
Always, always a darkwraith. Never a darkmoon. Never. Darkmoon invasions way too easy.
I use completely different builds for hosting or invading. So for the hosts I will have as many bosses alive as possible, and the invaders every boss except Gwyn dead.
Ceaseless isn't worth keeping alive unless you want to host a fight club there. I keep gargoyles alive at times just so I can have low-level greifers invade my level 200. And there is pretty much no activity in Blighttown. I can say that I have never once been invaded there, so just kill Quelagg.
At level 100 you will mostly just get duels or ganks instead of random invasions in most places. Level 99 to 125 is the level you want to be for duels when you invade, but hosting you can do at pretty much every level. Might be less activity though. If you go to the forest you can pretty much host, invade, or do darkmoon invasions at any level and you'll get matches.