WetGeek Budgie seems to be the one that's being put forward as the default - it appears first on the download section.
I know it's a newer desktop and not as mature yet, but I think Solus is selling itself short by doing this. IMO MATE is a better choice - Just look at Linux mint. It became the most popular Linux distro largely due to the MATE desktop. It's fast and runs well on reasonable hardware.
It surprised me that Budgie ran so slow because I do have a fairly beefy machine - desktop with 32GB i7 Coffee Lake processor and pretty good graphics card.