I've been working through a couple rolls of Kester #30/245 0.020", which is 63/37 no-clean. I didn't get that for any particular reason, rather it was cheap on eBay. (And by "working through", I mean I got three rolls like 10 years ago, and I'm finally on the last one.) It's a bit less reactive flux than generic types (RMA usually?) but that was easy enough to adjust to. I do use a little bit more (external) flux than otherwise, I think, which is fine, probably better to be in the habit of doing anyway.
Alloy isn't too important, 60/40 is very common, 63/37 is just a little nicer to use, and add Cu, Ag, Sb, etc. if you need a stronger alloy.
The diameter is a bit small for bulk purposes, but, also easy enough to use some scrap solder blobs and paste flux to handle those cases. It's plenty fine for SMT.
Actually SMT isn't that big a deal for diameter anyway, as you don't often apply solder wire to the joints, you're often tinning the tip, then the pad or part. You have three chances to meter the solder: quickly dabbing the wire, knocking/wiping off the excess, and what actually gets into the joint.
Especially with a hot air machine to help out (a crummy one is like $50, why miss out?), all you need is tinning the pads first, extra flux, and hot-air the parts in place. Solder paste is somewhat a luxury, but also quite affordable and usable.
For stuff like 0402s, personally I prefer hot air, and smaller pads. Small pads (around IPC-7351B high density) are less prone to tombstoning, and the part centers more strongly. Even with large pads, I find it's hard to get a soldering iron against both pads at the same time, and it's almost always going to involve too much solder. A wedge tip does help. Instead, I've been using hot air, and a fine bent cone tip for touching up pads as needed.
Hot air can be pretty slow (particularly with inner planes), but I find the patience is worth the consistent results, versus the potentially frustrating struggle: whether I've got enough solder on the tip, or held at the right angle against something I can't see, or endlessly fumbling with a tiny chip that wants to float on the solder blob instead of stick to the pads, etc...
Tim