Robot from the Future
I don't know what calcs you were doing (assuming 15IVs [I think E4 have 30's? I don't remember]) or what you were testing... Probopass doesn't even have Earthquake...Give me solid evidence as to why it's the best by a wide margin and then i might believe you.
So far this thread has failed to do that.
(Colonel M: I tested. Delphox can't tank a Power gem or an earthquake)
Probopass is Lv 63 and Delphox is Lv 60 for these calcs. Granted, you aren't exactly doing shit back, but Delphox can Calm Mind (as its way faster), Calm Mind again, heal, Calm Mind again -> Death to Probo
0 SpA Probopass Power Gem vs. 0 HP / 0 SpD Delphox: 90-108 (53.2 - 63.9%) -- guaranteed 2HKO
0 SpA Probopass Earth Power vs. 0 HP / 0 SpD Delphox: 68-82 (40.2 - 48.5%) -- guaranteed 3HKO
If you Calm Mind up Vs Klefki (as Colonel M said)...
0 SpA Probopass Earth Power vs. +3 0 HP / 0 SpD Delphox: 28-34 (16.5 - 20.1%) -- possible 5HKO
0 SpA Probopass Power Gem vs. +3 0 HP / 0 SpD Delphox: 36-44 (21.3 - 26%) -- 1.7% chance to 4HKO
+3 0 SpA Delphox Flamethrower vs. 0 HP / 0 SpD Probopass: 112-133 (66.6 - 79.1%) -- guaranteed 2HKO
Delphox is more than fine in this matchup.
As for the other two starters: Chespin fails to make a huge impression at most points in the game. It's reasonable against Viola simply because of Rollout, Can take on Tyrunt but not Amuara, does okay vs Korrina, can resist and go toe to toe with Ramos (but Acrobatics hurts), it does well vs Clemont, and outright loses to the final 3 gym leaders (all hit super effectively). So so performance in the E4 too. Beats Seibold and does reasonably against Wikstrom, but doesn't really touch Drasna or Malva. It also loses most matchups with Flare as they prefer Fire/Poison types, but it can hit the Dark types with STAB later in the game.
Froakie starts off very frail, struggling with the gym he should be SE against (Grant), gains Surf after Korrina (which it painfully needs), outright loses to both Ramos and Clemont, will likely lose vs Valerie, beats Olympia and then does okay vs Wulfric. Greninja pulls Froakie up the order somewhat with a larger movepool expansion and being able to learn Ice Beam. It's base coverage with just Dark Pulse/Surf/Ice Beam let's it beat everything in the E4 apart from Siebold (where it also holds its own). It's issue lies in the midgame however, where Frogadier does not really help in a lot of situations.
EDIT: So IAR has told me that E4's have 25 and Diantha has 30 for IVs. Makes a little bit of difference to the calcs (around 7~10hp). Nothing massive though (thats 25 for Probo and staying at 15 for Delphox)