Which race has won the most = how long is a piece of string
All comes down to the players in the games i think.
Whats is a cloakers worst nightmare, well that depends on what cloaking race you are.
Birds and Liz would be ion storms/poppers/minefields/webs.
Pirates would add Loki's to that list
Klingon would add loki's and delete poppers.
It's one thing to try all races, but imho you need to assess what type / style of play suits you best. In my case it's the sledgehammer approach, so i tend to do better by far with big carrier races, my favorite being the Borg.
Playing the birds or pirates takes a lot more planning, stealth and patience for most players. I accept those that specialize in these races can do things faster, but for the average player, it takes a lot more thought.
As for how aggressive to be, well that depends alot on who your neighbours are, and who you are.
Best to read the strategy section for all races, they give hints and tips to who are your best allies, who are your worst threat and how to deal with them. Get an overview of your neighbour, for example if you are the birds or lizard and find very early the borg are your neighbour, then go in for the kill as soon as possible... it take the borg quite some time to wind up, but once they have you cant stop them.
I think for the best players around there is no set strategy apart from their economy. Once the neighbour is detected, then they would decide on whether aggression-NAP's-alliance or stealth is required.
Started playing in '95 and still have yet to start a game as birds/liz/klingon/pirates save for altered scenario games like SFB. I have taken games over with these races, but you are then stuck with someone elses 30+ turns of tactics, and often a c-players erraticness for some turns as well.
Well those are my thoughts, don't know if they actually help or not.