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.
I agree with Stanwal on this. If you can replace with a Rega, Project or Music Hall that would be a nice step up.

Some will say Technics 1200 but I personally favor belt drives.
Thanks. Will both the Rega and the Clearaudio play nice with the Ortofon 2M red?
What is it you want to improve--what is it you're not hearing that you want to hear?

Does the turntable have built-in interconnects or are you using 3rd party interconnects? What is the turntable sitting on? Do you have an isolation platform or vibration controlling footers?

How carefully have you aligned the cartridge? Have you checked it against a good protractor?

Is your turntable level? Is is far enough away from the speakers to avoid feedback?

Maybe a record grip or damper puck would help?
Keep the 45F. The difference between a 45F and a Rega is not worth the loss of all the automatic features of the 45F, assuming your set up of the 45F is optimal. I would change the phono preamp to get a better match with your cartridge. I have a 47F running to a Marantz SC9 and also a VPI Aries. Yes, the Aries is better since it has a $2K cartridge and SDS box, but the 47F is also great and it has all the auto features.