My ideal team wouldn't be an ultra-competitive balanced type team, but a team of the six Pokemon I have the strongest attachments to. In that case, it would be:
Sceptile (First ever Pokemon, Solo'ed basically all of Sapphire)
Torterra (My first Level 100 Pokemon, and my first Pokemon to beat a Champion)
Articuno (My main Pokemon in the Blue Rescue Team postgame with maxed IQ, a move that hit every Pokemon in the room, the game-breaking-in-PMD Agility, and a linked Mind Reader+Sheer Cold for instant 1HKOs.)
Giratina (My strongest non-sidegame Pokemon and one of the only Pokemon I've used over multiple games.)
Illumise (My starter during a randomized FireRed run that I used in my first time emulating a game, mostly as a test run.)
Gyarados (My ace during my mono-Flying Red run that got me back into Pokemon after a long absence.)
I should note that these are all specific Pokemon from specific playthroughs. It's impossible to bring more than half of them to the same game, and even then I'd rather leave them in their original games. My dream team couldn't just include any old Gyarados, it would have to be this specific one, for example.
If I had to pick an ideal, non-specific team, it'd be Gourgeist, Chandelure, Mimikyu, Sableye, Drifblim, and Polteageist.