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.
So I've taken the advice of many here and bought a Era Gold here on A'gon. I'll report back when I get it and I can A/B with the pro-ject.
Congats on a fine choice. In addition to the above and perhaps to highlight some of the above implications, I would emphasize that saving up for a better table/arm would definitely yield worthwhile benefits, although you may need to up the ante. I would also add that a table/arm upgrade will give more than that of a cartridge. Of course, it would be that much more worthy of a cartridge upgrade. IMHO, the Rega are the best of the forementioned contenders but perhaps not enough of a step up. If you're curious, it's likely to be not too difficult to get hold of a P5, which would be slightly more likely to be an improvement, especially if you can audition it with the outboard power supply. Then you can hear for yourself just what kind of sideways move you're talking about. Sometimes a different flavor of icecream can be very educational. Of course, a P7 or P9 would give a more refined taste of the Rega option, especially as a reference for other higher end aspirations. As a Rega dealer, my point here is not to promote them so much as to answer your question, or even more to the point, to encourage you to put yourself in the position of answering your own question. You may even be able to bring your table in to a dealer and compare it there.
So I had the Era Gold in the set up for a few days and it does make a noticeable difference over the Pro-ject. I'm very pleased with the set up as of now. I'll probably enjoy it for a while then look to making an upgrade in the TT.