Gen 4 by far especially in its heyday
Gen 3 purists will claim Gen 4 introduced many things that ruined the meta, and to a certain extent they right, but they aren't so abusable in Gen 4 plus most of the tech you see nowadays wasn't used back in 07-09 since people are just now discovering that some moves useful in current gens can still be useful in previous gens (eg: knock off, which was never used before Gen 6, is now a staple of earlier gens too). Gen 3 is also pretty great though
Any metagame with team preview is a boring piece of shit for me. I can instantly tell who is going to win or lose the game based on team preview and there's less room for more interesting and dangerous play
