It completely depends on the type of games you wish to play and how much time you have.
If you have the attention span of a gold fish and think like a woman then the Wii is perfect.
If you love long RPGs (60 hours+) with some great storylines and combat systems then you can't beat the PS2.
If you want to experience some of the best gameplay and innovation then get a Dreamcast.
If you love strategy, FPS or online games then go for a PC.
Each system offers enough difference in the genres they excel at to make them all worth owning at the same time.