With the new balance changes coming, the question can apply to both CW and the 9k PS mode. I will include my guess below.
CW: Masto Spider, Punny Spider, Scorp Hover/Harvester tied for 3rd
9k: Draco, Harvester, MG Spider
I think CW will quickly grow stale, with a lack of build diversity. I think 9k will have a ton of diversity, making it the most enjoyed mode.
9k: Diagonal destructor photon/deadman wheel build, Draco wheel build, Parser banana. Gotta add a fourth tho. Bigram gremlin ermak.
The last time they had Arena, I played Dracos and had a lot of difficulty. The format is so fast that hitscan rules and it is hard to get cars to slow down. I had most of my success with a little machine gun spider.
For this mode I will try to develop a triple Porc build of some kind, maybe with Omnibox and 4 Sleipnir tracks, a little machine gun spider with some combination of Nothungs, a Vindicator and protectors and a 2 Stillwind fast little hover. That should cover my bases for most situations.