I tend to put it after delay... reason being that it would be there if it were the amp's tremolo.
I also find that trem (or chorus or anything else in the modulation camp) can get chopped up funny in the repeats if it's running into the delay. After delay the tremolo stays consistent, as does your delay's decay. When using a volume sensitive delay patch, for example, it's much smoother and more predictable to have the tremolo's varying volume levels after the swell effect.
I normally put the delay last, but I'm finding with my Line 6 Tap Tremolo, the buffering of the trem signal by my DD-3(even when bypassed) gives a "flat" quality to the tremolo. It sounds much better when it's last in the chain before the amp. Plus, I don't usually use delay with tremolo anyway.