Same for me. I do best on Carbon and Bodog. I think it's because both of those sites have casinos
attached and you get "gamblers" drifting over to try their hand at poker. But one thing I notice is, once a site gets a reputation as fishy, especially if it's a smaller site like Carbon, more solid players start moving in and, after awhile, it's not so easy any more. I play low stakes at Carbon and am seeing the same players over and over; they're not there to lose money.
I've always had trouble on FullTilt. It's not that the players are trickier, it's just that many are tight. I think there are a lot of multi-tablers who don't speculate or chase much. I found it really hard to get paid off for big hands on FullTilt, and could just never get ahead there.
Having said that, every table is unique. Every time you sit down, regardless of site, you have to get a feel for the table. Are there action junkies who just love pushing pots whether they have a hand or not? They tend to infect a table and make it very expensive to take any draws. My favorite players are, of course, the loose passive ones who never raise but who call a lot with speculative hands. At some tables, you can three-bet the button then, if the flop is unraised, take the pot 90% of the time with pot-sized bet. Just a month or two ago I was doing that quite a bit at Carbon. Now it's not working as often. Either they are on to me, or I'm running into players who are capable of trapping or, possibly, action junkies.
At some tables, you might be able to do a lot of limping if you like that sort of thing (why would you?). At others you need to be bolder if you want to be in the game. Bottom line is to just adapt to the table conditions wherever you play, don't get stuck in a rut. Pay attention to what's going on and figure out how you're going to take advantage of it. It's a mistake to stick with a certain style, or to even try to develop one.
But I do like to pay where I seem to do best which, for now, is Bodog or carbon.
Keep in mind, though, I'm probably not a very good player. Usually when I post advice I get a lot of people correcting me saying "that's not quite right." Once I get my ego out of the way, I pay attention and try to learn. But I never get anybody saying "great post!".