There are probably more titles that I wish I had bought when they came out than the ones that I got when they came out. Obviously there are lots that I could never have got, because I wasn't born yet. But I had opportunity from the late 1960s onward to get titles and I didn't--which I kick myself for now.
Never got most of the mod Diana Prince comics (bought all of them decades later, mostly from eBay auctions); never got the GL/GA issues with Adams art except for one issue (the John Stewart issue) and I still haven't managed to get all the back issues; never bought the early issues of STARMAN (I only have the tradepaperbacks); never got most of the Fourth World comics--I've filled in some issues, but many more to go before I have them all; and lots of horror, western, war and romance comics that I completely overlooked in favour of super-heroes back in the day.