Hillary of course.
but she wasn't the first 1-billion-dollar nominee. that was the dude that told us how bad trump is, last night at the convention. So did biden. Eff, 2020 saw at least 14.4 million spent. and thats not counting the duel charities/pacs like planned parenthood and Human rights campaign
https://www.opensecrets.org/news/2021/02/2020-cycle-cost-14p4-billion-doubling-16/
Democratic candidates and groups spent $8.4 billion, compared to $5.3 billion for Republicans. Democrats' spending falls to $7 billion when excluding spending by billionaire presidential candidates Michael Bloomberg and Tom Steyer.