We have started a small biz LLC owned jointly by me and my husband (50/50). We file joint returns. What is the best way to record our initial investment into business?

Since you elected to file for an LLC, you have to also file as a partnership. Which means a partnership agreement registered with the state, the state sales tax office and the IRS, and you file using the form 1065 at tax time.

This explains joint ventures for married folks, but notice it says very plainly that you can not be an LLC and get this tax treatment.
https://www.irs.gov/businesses/small-businesses-self-employed/election-for-married-couples-unincorporated-businesses

regardless each of the owners/partners should have a set of equity accounts, I suggest for sole proprietors and partnerships the owner/partner equity accounts look like this:

[name] Equity (do not post to this account it is a summing account)

>> Equity ( first of the year roll up drawing and investment into this account as well as retained earnings)

>> Equity Drawing (record the value you take from the business here)

>> Equity Investment (record the value you put into the business here)

So to finally answer your question, you use [name] equity investment as the source account for the deposit you make into the business