Wow, that's a really hard question to answer. But, I think you kinda answered it in a way. I'd say GH is the "base" peptide as test is to aas.
You've basically got three main angles to hit systemic growth from:
1) Sex hormones, i.e. testosterone
2) Growth hormone
And each has their own "branch off" drugs, or drugs that compliment the "base" drugs:
1) You've got less androgenic steroids like Anavar, winstrol, decca, and so on. But, now you've got SARMS which are still effective and even less androgenic than the least androgenic steroids. During a cycle I'd use test and another less androgenic steroid of your choice, but off cycle I'd use a SARM to retain gains and strength. They're great for that.
2) You've got the growth hormone releasing peptides and factors- ghrp-2, ghrp-6, ipamorelin and you can comine any one of those three with a grf like mod grf 1-29 or cjc-1295 w/out dac for a synergistic effect. The combo of one of each of these will ellicit a good gh pulse, but it's even better used with gh- taken a short time after the peptides. I'll never go back to using them separately if I can help it!
3) Insulin. Well, igf1-lr3 is systemic just like fast acting insulin. So, you get a lot of the same effects. It's up to you which one you want to go with lr3 or just use insulin. Either way you'll see the best benefits when combined with a good steroid cycle- same with a gh protocol.
After that you are talking about fast-acting igf-1 drugs that are more localized. That is a whole other topic because they aren't going systemic so much, more of a site-specific effect in hopes of producing new muscle cells. In my opinion, mgf, igf-1 rh, and des (1-3) igf-1 are all "icing on the cake" kind of drugs to be used by those who have already built their "base" of solid muscle and want to add more peak to their bis, size to their rear delts and so on. They're potentially effective/decent drugs imo. But, they're as far from a "basic" drug as you can get since their purpose is so specific.
I guess what I'm trying to say is that there isn't a "first" peptide to use unless you count GH maybe. You'll see better recovery, joint health, and fat loss from GH alone, but that's about it. You want a semi-legal muscle builder? Use a SARM. Want a semi-legal gh drug? Use gh peptides. It's all about what your needs are.