I'm with Heartland; I don't have too many games on my backlog that I've played a bunch, but have never finished (in fact, I believe the majority of the unfinished ones, I've never gotten around to even playing them!)
And I don't, "regret," anything (I mean literally in life; there is NOTHING I regret, as everything is either a good time or a learning experience
), but some games I should get back to completing before others, include:
32X:
Kolibri: The only non-beaten 32X game I have where I was right near the end (think the 19th or 20th level). It's an awesome title, btw.
Dreamcast:
Carrier: I was about 25% into it, then I saw the Prima guide on eBay for like 5 bucks, so I put it aside and waited for the guide to come in. It took like 2 months and I haven't gotten back to it since (this was in 2009, btw; LONG ass time).
Illbleed: Same thing, only I had just completed the first level and never went back to it. Superb and underrated title.
Rayman 2: 25.4% completed it in 2007 and haven't gone back to it since. Criminal of me, as this is a phenomenal game.
Genesis:
Vectorman 2: I wrote about it mid-last year, that I couldn't beat it at the penultimate or 3rd-last level; I will give it another shot.
Zany Golf: I've been playing this game since I was 3 or 4 and I've only ever made it to Energy Enigma three times; my most-recent time playing (a few years ago), I made it just close to the hole, but was ultimately a shot or two behind what was required
GameGear:
Ninku: I spent so much time on this, getting past the midway point of the game, then hit a wall; I didn't have a guide at the time, so didn't know what to do, but surely I can complete it now.
Saturn:
Burning Rangers: Got to either the 3rd or 4th level (can't remember and never picked it up after that... and I'm not sure why. It's brilliant.
Soviet Strike: Same thing as many above; got to the penultimate level and that was it.
Master System:
Alex Kidd in Miracle World: Yeah, yeah; I know. I just can't beat it.