looking to buy which i mac has 6gb VRAM or higher..

Macs no longer have separate memory for the CPU and GPU. They use unified memory. The following has a description of that: https://www.macrumors.com/guide/how-much-mac-ram/

Topaz recommends an M2 or later Apple Silicon Mac with 24GB system RAM.

from: https://docs.topazlabs.com/photo-ai/system-requirements

Is your question really about which Mac has the most powerful GPU? That depends on the processor generation (M1, M2, M3, or M4) and level (plain, Pro, Max, or Ultra).

Among M4-family desktops, your choices are:

24" M4 iMac

• Plain M4 with 8-core CPU and 8-core GPU – 120 GB/s memory bandwidth
• Plain M4 with 10-core CPU and 10-core GPU – 120 GB/s memory bandwidth

Mac mini

• Plain M4 with 10-core CPU and 10-core GPU – 120 GB/s memory bandwidth
• M4 Pro with 12-core CPU and 16-core GPU – 273 GB/s memory bandwidth
• M4 Pro with 14-core CPU and 20-core GPU – 273 GB/s memory bandwidth – custom-order option

Mac Studio

• M4 Max with 14-core CPU and 32-core GPU – 410 GB/s memory bandwidth
• M4 Max with 16-core CPU and 40-core GPU – 546 GB/s memory bandwidth – custom-order option

Your best bet IMO would be a Mac Mini M4 Pro with 32 GB RAM, 1 TB SSD and a 4k monitor, either 27" or 32". That configuration would be at least $650 less than a similarly configured 24" M4 iMac.

Note that the linked Tooaz page has separate system requirements for Intel-based Macs and for Apple-Silicon-based ones.

All current Macs use Apple Silicon (M-series) chips, so when shopping for a new Mac, you want to be looking at that “tab.”

+1

Why do you need an iMac?