It's not that it gets better, but it certainly never drops quality: Super Mario.
I would say Grand Theft Auto. Everyone has their opinions but you can't deny the strides made from SA to IV to V in terms of physics, graphics, mechanics and performance.
The fact that you forgot to mention the fourth game says a lot though...
If it weren't for Sunshine, I'd say the Super Mario series would be the perfect answer, since it got better with every entry. But then... what does "mainline" mean, in the case of Super Mario? Handheld titles don't count?
But San Andreas and Vice City isn't betterCertainly not GTA, since both San Andreas and Vice City are better than IV.
A few months ago could have added Tetris on this list...
I'd say Civilization, although its always an "...eventually" type deal, as the next installment vanilla release has been a weaker game than the previous one with all expansions and patches included.
But San Andreas and Vice City isn't better
Animal Crossing
Wow wrong on every countAdd Elder Scrolls and you got a deal.
Nah, it really doesn't count because of the constant arbitrary addition and removal of features and the unoptimized games.For those of you saying Pokémon it doesn't count since it is an annual series.