Aside from legendaries, the starters have always been hugely reliable, of course. The Charmander and Torchic line, primarily. Gardevoir on many occasions has ended up a very reliable psychic, and Vaporeon in general is always a fall-to water type, considering most ROMhacks contain Eevees.
As far as "dream team" status is, which I can't always manage in every game, Zekrom is a commanding dragon/electric type, easily reliable. Groudon and Kyogre are really good water and ground types that I can easily count on to do their job for the team.
Deoxys is a HUGELY reliable sweeper, especially in attack form. I found I can almost always rely on those massive offensive stats and the decent movepool Deoxys can have. Give Deoxys a moveset that super-effects most types, and it'll destroy almost any team on single player.