Which is the only metal to burn in nitrogen?

Lithium is the most reactive metal

Due to the sheilding method, Francium ought to be the most reactive, shouldn't it?

Lithium, magnesium and aluminium are the only metals that will react directly with nitrogen at room temperature and pressure to produce the nitride.

Note that in air any metal that reacts with nitrogen will very quickly tarnish with a layer of oxide which prevents the surface layer reacting further with nitrogen.

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Actually, I may have been wrong there as lithium, sodium, etc are usually stored under oil to prevent the air eating them away.
As you say the aluminium oxide layer is so attached uniformly to the surface that aluminium comes across as generally unreactive.

The alkali metals (Group 1) increase in reactivity DOWNWARDS. Therefore, lithium is the least reactive of these elements, while francium is the most. Try it yourself in a lab. You will find that potassium burns far more vigourously than sodium, and sodium more so than lithium.

In answer to mountside's question, I'd go with Francium.

It's Titanium.

I realize this is like, more than 2 years old, but... You guys are seriously simplifying reactivity. Example: aldehydes can be reduced by NaBH4, while carboxylic acids cannot. Yet carboxylic acids can be reduced by BH3, while aldehydes cannot. Every molecule is prone to certain reactions more than others based on their properties.