Should I get a new cart, TT or phono stage?

At some point i want to replace all three but I'm wondering what will make the biggest improvement first.

I have an old Denon DP-45F with a recently purchased Ortofon 2M Red going into a Pro-Ject Phono Box II which then goes into a Simaudio i1 integrated.

What would you upgrade first. Some things i was thinking of were a Rega P3, Clearaudio Emotion, Simaudio LP3 or used LP 5.3, or a ortofon 2M Black.

If I made any upgrade i would likely be stuck with the remaining components for some time before I made the next upgrade.
Thanks