When looking at US gold coins both pre 1930s. Gold Dollar Quarter Eagle Half Eagle Eagle Double Eagel Then further Liberty Head or Indian Head
Back when they circulated, 90% of folks couldn't afford to even carry a gold dollar, never mind a double eagle. I'd say none were very popular then. Now? I don't have any idea.
the $10 & $5 were the most popular in the west where paper money was distrusted. the gold dollar got practically no circulation because it was so inconveniently small, the $2.50 was occasionally circulated, the $3 was only used to buy postage stamps, because the denomination wasnt good for much else and it was so close in size to the $2.50. the double eagle circulated mainly in california, and was a lot of money back then, so many were never released and melted in the 1930s. now i'd say the most popular is the St. Gaudens double eagle.
You could probably figure this out if you analyze the price trends for each of these series. That said, off the top of my head, I think the larger coins, particularly Saint Gaudens double eagles, are pretty popular. Keep in mind, many of these coins are purchased by investors and not simply collectors.
You didn't specify so I'll add to your question "... for collectors". NOTICE that I'm excluding bullion investors. Now I'll respond IMO it is Saint-Gaudens Double Eagles.