I've only got three coming to mind for me right away, one that's canon, one that's not but I believe there's evidence of a one-sided thing at least, and the last is entirely out of my own (and a few others) mind.
First up is Gohan x Videl. This one is the only couple that has any real lead-up in both the anime and manga if you ask me. Goku accidentally promised to marry Chi Chi, and then she pretty much never shows up again until years later in both versions. Vegeta and Bulma is the biggest plot contrivance ever (even if it does work great when you examine it). Even Krillin and 18, as adorable as I find it, is pretty out there and is a sad lead-up. Gohan and Videl though, we see some great interaction between the two way before the question of romance ever gets brought up, we see Videl slowly start to fall for him (especially in the anime), and then when it finally happens, it's totally believable. No other couple in DB has this kind of believable build up, at all.
My second one would have to be Goku x Bulma. I'm in the camp that honestly isn't that fond of Chi Chi. I get why she's the way she is, I really do, but I just can't bring myself to like her that much. She is a good foil for Goku, and likely the only one that could put up with him...but I've often pondered how different things would have worked out maybe if Goku and Bulma had ended up together. If the series had been done by anyone else but Toriyama, it'd be a perfectly valid idea as things went along after all. Bulma was the one who changed Goku's world, she was his first friend (and I am a sucker for friends-to-more romances), and we see at least once in the manga and at least twice in the anime that Bulma has a slight physical attraction to Goku later on. Granted, that's just her being her usual 'this guy is hot, I like him' self, but still. It's just a couple that I like to think may have been a fun one if it had turned out that way. A missed opportunity perhaps.
My final one is Pan x Uub. There's absolutely no evidence of this anywhere in GT, but GT being what it is...yeah. It works really well as a pairing if you ask me, if given the proper build up. Goku leaves to train with Uub, and I for one would like to think that eventually, after he's gotten Uub up on the basics, he'd start to bring Uub around to his own home to train there instead of staying in Uub's village forever. When he does that, I'd assume he'd start training Uub and Pan side-by-side. Pan would likely be annoyed with Uub at first for 'stealing' her grandpa, so their relationship would be kind of a Gohan/Videl situation, how I imagine it. That and there's something oddly fitting and romantic about Goku's granddaughter and his pupil hooking up. I've also never been a fan of Pan/Trunks, even without the age difference, so this one just gels with me.
That's all I got for now, so chip in guys.
C'mon, I know you want red haired half-saiyans too!! 
I just see it as something that worked out far better than Toriyama originally thought it would, or even put that much thought into it. It's start is pretty much just 'Oh crap, my idea involves another Super Saiyan, a new character. Crap. Who can I have spit out a kid of Vegeta's? ...Well heeelllloooo Bulma'. That hurts this pairing a little for me, but it's still a great one.
I've heard of a few Piccolo x Videl supporters, but this one surprised me to be honest. I...really can't say I see this one. Even by the Boo arc, I don't think she's particularly warm towards him, not to mention the whole asexual Namekian thing, so...yeah. I'd pair Piccolo with nobody. 
I think she'd be a great optional new option for Goku just like Bulma. ...There we go, forget Chi Chi, Goku comes to the tournament to fight Piccolo, and the love competition/triangle with Goku/Bulma/Suno can begin!
" 
And there's a big assumption going on that Freeza might had had his way with Vegeta at one point. 
Not pretty.