I"m looking at line editing, as they can pick up if you are overusing certain phrases, but I can't afford any of them at the moment!
Hm, that's a tough one! Though I'd say avoid proofreading as, unless it's a final draft, that can be a bit of a waste (if things change grammatical issues etc., can get introduced and another round of proofreading would be necessary). If it's your first time having someone look at it, or even someone professional, developmental is probably the way to go. An agent or publisher is probably more likely to pick up something that has a great story/well developed characters etc. (all things you'd get feedback on with developmental edits) but needs a bit of a polish, than something that looks and sounds nice but has more inherent issues. In a typical format, developmental edits come first, line edits second, proofreading last. That's my advice at least!
Thanks everyone for your comments. They're very helpful.