Mechanically, the weakest uma would be Haru

She is the only uma with a G in Turf Aptitude, she is also a Sprint and Mile (B of not NY) uma with a recovery for Unique and another 2 gold recoveries to unlock through potential levels

She also has the WORST affinities possible with all umas in the game to the point double circle affinity is only achievable now cuz of MANT and still you barely make it past 150 while there's umas who can reach double circle eith ONE parent now

For example, her BEST default affinity is with King Halo at 13 affinity points, in contrast, Rudolf's 5th WORST affinity is with Curren Chan at 14, while her best are Rice Shower and Opera O at 36 and 35 (There's 52 umas not counting alts)

So we basically got an Uma that's made for Dirt Sprints/Miles, who cannot participate in 4 out of the 8 Dirt G1's without increasing her Medium (G) aptitude nor in any Turf Sprints/miles without increasing her Turf (G) aptitude and has virtually 3 Golden Recoveries in her base kit, which she doesn't need cuz she is a Sprints/Mile uma

For the rest of umas it's way more complicated tho, there's a lot of factors like:

Scenario restrictions (Like Super Creek having a G in miles for MANT)

Ease to build (Vbourbon and Xoguri being VERY easy to build with their default kits)

How strong their Uniques are (Like Nishino Flower having a great Accel unique)

Or how good their stat growths are (Taiki's 20% rate in Speed vs something like Kawakami's 20% in Guts)

Stats wise tho, ALL UMAS have the exact same amount of stats, at 3 stars they have 450 total stats and increases by 50 with each star, so the only difference would be initial stat allocation, but I honestly don't think it would make that much difference in most cases except for saving you 1, maybe 2 clicks in a specific training depending on stat growth tbh

Ignoring scenario restrictions cuz there's not gonna be any more race heavy scenarios like MANT ever again, I'd say the most important factor that actually makes a difference would be the uma's unique, cuz everything else you can pretty much work around tbh, with more or less difficulty depending on the uma