It's so individual, it's really hard to make a definitive statement. If you want 3 kids and you're 39, you can't really "afford" a four-year spacing, you know? Or if you want your children really close in age, you might not get your fertility back for 2 years between each one and just have to wait it out. So I don't think there's a single "best", at all. I come from a family of six, spaced thusly: 1 1/2 years between #1 and #2, 1 1/2 year between #2 and #3 (me!), 3 year gap, then a 6 year gap, then another 3 year gap (and a miscarriage a few years after that). I'd say there were pros and cons to
all those spacings.
Some facts I took into consideration before TTC:
- They say it takes two years after childbirth for a woman's body to regain its "normal" state - I think it's related to mineral stores, vitamins, hormones and so on. I'm not sure how breastfeeding factors into that.
- 70% of women lose their milk supply altogether during pregnancy. TTC at one year would indeed get you out of the formula zone, but it still might cut your nursing relationship short if you're planning to EBF. Of course, there's a chance you might not lose your milk, or your LO might happily nurse through anyway: but it's worth considering. Night nursing during pregnancy can be tough, tandeming can be tough, and so on.
- From many and various MDC threads on child spacing, I've noticed a LOT of women prefer a 3-4 year age spacing. The idea is that the first child is well out of the "baby" stage - often nursing minimally or not at all, playing with older-kid toys, able to help a pregnant mother rather than hinder her and so on. So the first child felt less supplanted by the second, and was also more able to understand and enjoy the pregnancy (and in some cases, the birth). Some also mentioned that when the baby was bigger, they weren't in direct competition for toys/activities/friends, and that minimised conflict. That said, plenty of people have said they enjoyed smaller and larger gaps as well. Lots of people said of small gaps that the first year or two was really tough, but the two kids entertained each other really well after that and were very close friends at the ages of, say, 8 and 9.
- I do Traditional Foods stuff, which places a great importance on preconception nutrition. So while DD was a "whatever" baby in terms of BC, this baby was preceded by some intense planning and eating.
We still didn't do a preconception diet for nearly as long or well as the WAPF would recommend, but I feel I've gotten a head start - and if I can't eat anything but crackers for the next six weeks, at least my body has some decent vitamin and mineral stores to fall back on. This isn't exactly about timing, but it is relevant if you want to do any kind of cleanse/take any kind of supplement you can't take during pregnancy/get your fitness levels up/fix dental work and so on.
There are heaps more factors, obviously. One very practical one I thought of was babysitting - my parents would be much more likely to balk at babysitting two toddlers or a toddler and a baby than a baby/toddler and a somewhat older, more rational and helpful child. We don't actually ask them to babysit DD that often, but it's lovely when we do!
So, for me, I initially planned to give birth to #2 when DD was nearly 4, with a few regrets that they wouldn't be closer in age. Then we changed our minds, and got pregnant a few weeks ago - DD's 2.5 and will be three-and-a-third when the baby comes. Only time will tell if we've made the right decision! Now the deed is done I'm actually thinking we should have waited a few more months, but for various reasons it was a now-or-not-until-much-later deal. So. DD's getting older and wiser every day, and I'm very gently cutting back on nursing (particularly all-night nursing - we're limiting it to going to sleep and waking up now, and after a few nights of rage she seems perfectly OK with it!). Next we need to work on gently transitioning her to a toddler bed in our room, and fully potty-learning her before the baby comes. She's already seen a bunch of birth videos, though!
Good luck in deciding!
The good news is, even if you get the spacing "wrong" you still get a baby. And that's nice.
). DH wants 3+ years before we start trying again, I'm happy with at least 22 months between births. I picked 22+ months because it would be 12 months of nursing the first. If I lose my milk immediately then at least I'll make it out of the 'formula zone'. 
