[ThinkBeat]

Delphi is the easiest and most integrated way of writing cross platform UIs and it has been for a lot of years. It might not be sexy, but highly productive.

You can download and use a community edition of Delphi for free. The downside is insane licensing costs if you are making commercial software.

I an not sure how far Lazarus has come but I believe it to have the same or similar abilities and if free and libre.

[phba]

I use Delphi at work quite a lot. The language is great, but the Embarcadero Studio is unfit for any non-trivial project and getting worse with every new version.

Nowadays I definitely prefer Lazarus.