Thank you all for your replies - and so quick!
I collect based on several factors and in this approximate order...
Priority I, essential
01. Gut feeling: totally irrational. Does the coin 'speak' to me?
02. Trust: in the seller, his/her reputation among numismatists and my previous purchases
03. Clarity and simplicity: I like purity of image more than beauty of image. I like geometry more than calligraphy, naturist symbols rather than script. I react negatively to complicated images. Give me one symbol done very well rather than six symbols and praise of the issuer done OK
Priority II, important
04. Beauty: I don't necessarily mean symmetry or even art in general. Difficult to quantify and qualify.
05. History of the type of coin: Is it a period of history that I enjoy reading about?
06. Lack of political overtones: I avoid images of monarchs and State symbols, preferring religious imagery (Buddhist, Hindu, other) - tree in railing, hills, swirls, conches, tridents, svastikas etc
07. Size: the bigger the better
08. Scripts: Indic scripts readable by scholars. I am fond of Brahmi and Kharoshti more than Devanagari.
Priority III, valuable
09. Value: other similar coins are selling for much more
10. Concentrated value: I prefer to have one coin worth $500 rather than ten coins worth $50, both for aesthetic and business reasons (the former collection is more portable and less work to sell, presumably)
11. Rarity: But, a beautiful large coin that is not rare is my preference to a coin that is ugly, boring and rare
12. Oddities: mystery republics and kingdoms, uncertainty as to what it even is or from where
13. Fits my collection: greater 'India'-focused. As much as I like coins from Kiev-Rus (and other Orthodox Christian States) or Hungarian (and other Catholic States), I don't want to spend a million years trying to sell or arrange gifting individual coins that don't fit. I would rather, find someone who in 12-18 years who wantrs everything I have collected. A matter of time-management.
14. History of the individual coin: provenance, ideally documented. Person A sold it on date B to company C., and as far down the line as recording is possible.
This is why I collect punch marked silver from the Japanpadas and other ancient Indian coins. I collect a few Mughal coins but generally have minimal interest in coins produced after the Puranic age.