If you mean giant robot transformation than i'd have to say "Gundam 00" and the rest of the Gundam franchise is pretty good.
If you mean like magical-girl transformation than "Sailor Moon" is a classic. Also "Full Moon wo Sagashite" is pretty good, although it's mostly about a girl transforming into a 16 yr old so she can be a professional singer...
Overall, i'm not too sure what you want- please be more specific w/ your question.
