Ralts: R,S,E,Pt,X,Y,OR,AS,US,UM (requires Island Scan)
You won't be able to get a team of 6 early in any game unless you prepare one in advance. (This is easier with more recent games.) Tyrogue has a 1 in 6 chance of being in an egg in Johto titles, but there's no way of guaranteeing you'll get it unless you're prepared to restart from scratch a few times. Snorunt is available late-game in a handful of titles, so if you're prepared to wait for it, you can work with a smaller team in the beginning and add it later.