Dear Dan,
I have used almost all the current Zeiss lenses on Leicas (15 - 2x21 - 25 - 28 - 35 - 50) and they're all very good. No adapters are needed. Leica and German-built Zeiss lenses cost more to buy because they cost more to make (e.g. hand-lapped focusing mounts, individual rather than batch testing, all-glass aspherics) but 'best' is hopelessly subjective.
For example, a 50/2 ASPH from Leica is sharper and contrastier than a 50/1.5 Sonnar from Zeiss, and has higher resolution and more even illumination; but I like the 'look' of the Sonnar better. Equally, I'd rather have a 16-18-21 Tri-Elmar than a 15/2.8 Zeiss (more versatile and a lot more compact, even though 1 stop slower), and I'd rather have a 75/2 Summicron than an 85/2 ZI.
I'd also suggest that what you can learn from an online comparison is limited. Original prints or projected trannies, or digital prints made using your own equipment, will tell you infinitely more.
Cheers,
R.