Is the gift card 1CD or a certain dollar value? If the latter, you can probably pick up two of the earlier solo albums for the price of one. I have seen Pictures at Eleven and The Principle of Moments discounted before.
He hasn't really released a bad album, but if I was to pick the one to buy first it would be Now & Zen. It's transitional between his early 80s anti-Zeppelin synth dominated sound and the later more rock and roll oriented material. As a bonus, Jimmy Page plays on a couple of the tracks.
I bought it when it first came out, and I have never tired of it. All the songs are very strong, and "Dance on my own" is really funny once you work out what it's about.