1.50 is the best firmware for homebrew (that can be achieved through downgrading).
The higher firmwares offer incentives for getting them like the Web-Browser, WMA playback, etc.
2.00 and up are capable of homebrew, but using special hacks:
2.00 requires the TIFF exploit
2.6 requires GTA save game hack(which requires GTA:LCS)
Depends on what you want. Personally, if I were you, I'd upgrade to 2.0, then downgrade to 1.5. Most everything seen on the higher firmwares can be used on the lower firmwares through homebrew.
Links2 browser released for 1.50, various media player programs to play more media formats, and now some 3d Homebrew games in the works.
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