I don't think there is one anymore. The range of breeding at the top level now shows that many different breeds can make it to the top providing they have good confirmation (usually) and the right temprement.
On a personal preference I would look for TB and TB crosses, warmblood - not those with the heavier build, anything with a little ID or even native particularly connemara. I do think you have to be open minded.
am open minded, my sister has a SFxTB who has is lovely to event and I have IDxTB but she is more SJ/dressage than event and my IDXTB was just a pain! but both probably would have evented!
I would get anything I could ride, aslong as it jumped and did dressage!!
My personal preference would be either 3/4 TB, 1/4 ID
or 1/2 TB , 1/2 WB although a modern warmblood (heavily influenced with TB lines) wouldn't e out of the question.
However I look more for a top quality allround sporthorse type to do all disciplines at lower aff level with, rather than a pure event horse ... if I were better and looking for 3*/4* potential I would probably want at least 7/8th TB
Tb with a dash of pony for me.
Boss is 1/2 tb, 1/2 connie.
Moon is actually 1/2 trakehner, 1/4 tb and 1/4 NF
Sunny is tbxirish sports, Floss is pure tb
My sister loaned an ex advanced event horse who was 7/8tb. He was lovely with beautiful paces and could jump (although he was extremely cocky). Although her 'proper' event horse was a 15hh tbxconn who jumped clear round a CCI* and was amazing. But he didn't want to push him above Novice.
Think if I was looking for a young horse I would go for a 7/8tb
Personally as much TB as I could get, pref more than 3/4's. But nowadays you need a touch of something else, either AA/SF/TREK/KWPN/ISH to add movement/scope. Pure breeds such as KWPN/BWB/ISH/TREK seem to do extremely well at lower levels (up to 2*) but then seem to lack the blood and stamina to break the 3/4star boundry, although there are exceptions in all cases.
Nowadays with the show jumping and XC less influential you need something that can do a really nice dressge test. So I would want something with a bit of warmblood to give it nice movement. Would need to be quick though to make the time.