Top non-green ramp spell? Probably
Stoneforge Mystic, and I'm not even joking.
Other colours don't really have a lot in the way of direct ramp, certainly not land ramp (
Kor Cartographer notwithstanding). Black has a load of mana doublers and the like which are great,
Crypt Ghast being the most obvious one, though these are a lot less good in 2+ colours. Red does too but it's more rituals and treasure generation which function as ramp but don't work in quite the same way as it's disposable.
I keep hearing good things about it but I'm worried about the discard part of it if I'm not playing self mill. How bad is it?
Compare this with, say,
Nature's Lore. When you cast
Nature's Lore you're throwing away a card (i.e.
Nature's Lore itself) to put a land onto the battlefield. Dreamscape is the same, you throw away a card in your hand to put a land onto the battlefield (you also sac a land and replace it with another land which can provide additional colour fixing). In a way, this is better, because you can choose from all the cards in your hand which to throw away.
Personally, I find this guy a little clunky. I have to sac a land and replace it which is annoying if I only have nice utility lands out that I'd like to keep, and can only tutor for basics. Discarding a card is, as I said, usually worth it, but can also be an awkward choice. Plus he costs three to activate and you have to pay 2 for him in the first place and untap with him, making his first activation like a very slow five-mana
Harrow. Eww. I don't even like
Harrow (though admittedly, this is blue, not green). You can play him T2 and activate T3, which is fine, but normally I'd rather play a 2-mana rock T2, and then already have that much mana, and later game he's even more awkward. I am often a bit too greedy with my manabases too so I don't even have that many basics often, though if we're talking monoblue, no worries, and even two-colour that should be fine.
All in all, he seems like a fair option for monoblue, but you want to activate him more than once and I think you'd ideally want ways to use the discarded cards somehow too to get the most out of him.