What is the best graphics card I can upgrade to?

Apple only offered two configs for your system:

• AMD Radeon HD 6750M, 512MB - Apple P/N 661-5944
• AMD Radeon HD 6770M, 512MB - Apple P/N 661-5945

Here is the IFIXIT guide to replace it: iMac Intel 21.5" EMC 2428 GPU Card Replacement

Unless your system has a bad graphics card I wouldn't spend the money on the slight improvements of the AMD Radeon HD 6770M card. Sadly, the size of the card as well as its heatsink used in this model limit you.

Frankly, you might find getting a used 27" 2011 model as a cheaper solution with the better graphic cards it offers. Or, upgrade it if you must.

• AMD Radeon HD 6770, 512MB - Apple P/N 661-5967
• AMD Radeon HD 6970, 1 GB - Apple P/N 661-5968
• AMD Radeon HD 6970, 2 GB - Apple P/N 661-5969

Unless you are working on images (pic or video) or a gamer. I don't think its worth jumping to the bigger video RAM cards.