Hmm... probably Devil May Cry 4 and Bayonetta. You get to keep all you got, can beat multiple difficulties at any time on the same save file, you can tackle other difficulties mission-by-mission and also play on other difficulties in-between (so you can get good stuff on an easier difficulty).
Hmm... yeah, a new game + doesn't really get better than that.
Ninja Gaiden 2 for the 360 also has an excellent new game + though. 1. If you load your endgame file, you start with everything you had (on the same difficulty though). 2. If you select a new game on a difficulty you've beaten before, you can choose to start with all weapons, ninpo and projectile weapons. 3. If you select a new game on an already beaten difficulty, you can also select a fresh new game.
Especially #2 is a rather unique and very nice NG+ feature. I'd like to see it more often. Start with all abilities, but not with any upgrades.
Read the mania: http://www.fanfiction.net/~nonexistinghero In SA2, it's Super Sonic and Hyper Shadow.