Tigana is fantastic - my absolute favorite.
The Sarantine Mosaic duology is incredible, though probably not for a first time reader of Kay.
The Fionavar Tapestry is an amazing trilogy with the books
"The Summer Tree", "The Wandering Fire", and "
The Darkest Road" - very high fantasy.
The Lions of Al-Rassan is beautiful.
I loved
The Last Light of the Sun
and
A Song for Arbonne is probably the worst of his work, but his worst is something that other authors strive, and fail, to do.
If Kays books could be put in any sort of order, from favorite to least favorite, that is probably the order I would put them in personally. However, it's pretty much impossible to do. My favorite of his books is the one I'm reading - meaning, every time I pick a different one up, it heads to the top of my list until I read another one - then that one takes it's place. A persons all time favorite of Kays tends to be the first book of his that they read.
For a new Kay reader, I would say start with
Tigana or
Lions Preferably Tigana, cause it really is my all time favorite
As a lovers gift it is good as well, as it's main stories are themes on love and memory...
course, the love theme can be found in all of Kays books