VBA-SDL-H should support GB/GBC games. I'm sure Ryusui will know more about this.

I don't know if VBA-SDL-H supports GB/GBC, but I know for a fact that BGB does.

BGB not only has some serious debug functions, but it also emphasizes accuracy. It was to the VWF I added to Sylvanian Families what I was to L. Ross Raszewski's "Moments out of Time: Adventure Type", with one crucial difference: BGB showed me what I did wrong a good six months in advance. I only got to MOOT after he released it, with much fanfare (and no small amount of anticipation from me; the first one was excellent), and despite its ambitions, the game is full of great big bullet holes: solutions that should be obvious are murdered by touchy grammar, and solutions that the author thinks are obvious are not because of inconsistency. It's not terrible, and it is nonetheless solvable, but it seems the author embarrassed himself a bit. (Though any game that so blatantly pastiches "The Journeyman Project" series is worth cutting a little slack. ^_^ "Moments out of Time" is probably the closest thing we'll see to a continuation of the franchise...)

...Random rambling aside, BGB features disassembly, tracing, breakpoints, a VRAM viewer, memory viewer, the works. VBA-SDL-H is probably the best weapon for GBA hacking, but it needs to be used in tandem with vanilla VBA, much as Geiger's Snes9x Tracer has to be used in tandem with VSNES: neither one has a proper graphics viewer, but that's only a small handicap considering what they're both capable of.

Wrong, no$GBA is better (if you have it :p).

This thread is about the GB though, so let's ignore the GBA for a while.

I can recommend BGB, while I haven't used it for hacking stuff I can say that the source was very well written and helped me with some GB emulator related things. As everybody else said, it's very very accurate where VBA takes some short-cuts. When I checked emulator behaviour against mine I noticed some of VBAs mistakes somewhere along the road, but I have forgotten the specifics. Not to mention that besides knowing this weaknesses my emulator couldn't even run one game in the end 

 

He must have one of the pay versions. The free one doesn't appear to have debugging options.
no$gb has some debugging stuff, but why's it look just like BGB? (and is he still charging for GBC emulation? )

Yes, go use no$gmb its the best gb/gbc debugger I've seen and i don't think he charges for it anymore after all of his releases were cracked by pirates to include gbc emulation and sound emulation when he had two separate releases for both sound and gbc

Well his emulators are usually worth it i know no$gba debugger was/is so is no$gmb though i never use it