So, I voted for last, but now I am wondering--do you mean garland or "icicles" (long, single strands of tinsel that you drape over the tree)?

I still absolutely say last for the "icicles", but garland... I dunno... I've never messed with it other than to wear it in place of a boa a couple of times. Not that I'm a freak or anything. Shut up. But if it is the big, long strings of stuff then I'd have to say put it on first then the lights and then the ornaments. Otherwise said big, long strings of sparklestuff will clog up the tree, mute the lights and obscure the ornaments... and quite possibly knock them off with the potential for breakage.

See? You're not the only with OCD, I am playing this out in my head as a hypothetical, therein applying too much thought to stuff.

We win at life, you know. *g*