Wii better compares to NES in most ways.
If you want to compare PS2 and Wii in terms of "greatness," as in, importance, then Wii will win. It is doing more to reinvent the market and draw in new customers, and it will probably outsell PS2, which makes it more important.
In terms of "quality software," which is subjective, so far Wii might be able to match PS2 for top 3 or 5 games, but not for top 25 or 50. I'm not convinced Wii will ever have the third party support of PS2, as it may take new companies rising up to fill the void left by current third parties' ability to make compelling Wii games.
No console will ever be considered to be the "best" by a particularly wide group of people. Right now, it seems most people say either SNES or PS2, depending on when they grew up and what types of games they like, but NES and PS1 were obviously the far more important consoles that SNES and PS2 built upon with upgraded versions of the same types of games.
I think it terms of which consoles are most "great" (important), it is pretty cut and dry: 2600, NES, Game Boy, PS1, DS and Wii.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.
