If I'd chosen by personal prefference, I'd have chosen 3
X Oddisey, I picked FE for the following reasons:
1) I know the set very well, so I can't really mess up too hard
2) It's small, so I'm juggling 102 cards instead of 300 for the first try. So it's much easier to tell if whatever change you make is working and how it's affecting the whole thing.
3) It's fairly complex, so if I can handle it I can handle something larger but more straightforward
4) Quest mode sets you up with random drafts, and if you want to choose your next draft you have to play and win whatever comes up. So they're all equal in terms of relevance in Forge, it's just a matter of when I get to one
5) Older sets have the worst drafting AI due to the bestiaire lists being inadequate and the highest number of easily fixable bugs, so they need the most help.
6) Playtesting it is easier on the eye due to art for me. If I were the kind of "only work on what I'm interested" kind of guy, Invasion would probably be the last set I'd touch, followed closely by Mirrodin. However, I have the best intention to give all of them equal treatment. I just decided to start at the beggining.
7) FE, other early sets, Oddisey block,
Time Spiral Block and Masques block first, as far as I'm concerned. Those were the interesting and weird ones. Of everything with the post-mirrodin frames, I think only Lorwyn, Innistrad and Kamigawa are really intersting, as for relevance - couldn't give a hoot. If I wanted to draft something recent, I'd be playing cardboard or MtGO. Not that I'd mind it working well in draft, but I'd prefer getting there from the other end.
And really, getting to draft MMA on Forge is meh, you don't need forge for that XD.
As for advanced archetype shennanigans, really, I'm currently only capable of making a big ole pick list. So Imma do that first. Then we see about more than that. And since I'm dealing with a set I actually know about, we can then see if stuff works easier