Explorer (Arena) list. Not sure how great the deck would perform in Pioneer, as I've never played it off of Arena/Explorer. I assume the deck wouldn't change much at all, though, as even in modern this is most likely the best 'shell' for an angels deck. Below is a bit of a long write-up for such a simple deck, but for players new to the format and returning players who haven't seen all the angels/cards then it might contain some useful info..
This deck has so much synergy it's disgusting. Almost every game I play (either bo1 or bo3) ends in the opponent's concession due to overwhelming (early - as soon as turn 4/5) board presence and life gain. Concession usually comes in the face of a bunch of triggers. I rarely HAVE to attack (vs. certain decks I have to keep up the pressure/clock, but more often than not I just sit back). I rarely even make blocks. ;) It IS that good on Arena, in Explorer, atm. Even in Historic the deck performs well (with addition of Soul Warden). It's fairly easy to play as well; It just about plays itself in most games/matchups.
Certain slots are flexible. Inspiring Overseer is the first creature to cut back on, Youthful Valkyrie would be next (or 1 Speaker of the Heavens). I guess the 4th Giada, Font of Hope is sometimes cut, but I would definitely recommend against it especially if playing all of the angels - ie. mono-white + coco list (I often play a 2nd Giada if I'm not too worried about over-extending, because it usually ends up entering the battlefield MUCH larger than the first one -- I've even played 2 back-to-back once or twice). Ajani, Strength of the Pride can be cut as well, although I have found this card to be extremely powerful. Many, many times I have played it as a 4mana one-sided wrath (that exiles and hits artifacts as well). Most of those times it probably was not needed, though, but against certain opponents it's amazing (which is why I still have 1 in the main rn, having moved the 2nd to the board - in place of what otherwise would have been a Settle the Wreckage). At times, I also play the deck with 3-4 Skyclave Apparition in the main (dropping some number of Overseers, Youthfuls, 1 Speaker, Ajani, etc.). Both versions of the deck work great. Prosperous Innkeeper is another card that is used as a 4-of in the main deck in some lists (sometimes including a couple Trelasarra, Moon Dancer as well), and according to stat sites it seems to work just as well as the lists closer to what I have posted here. There's yet a third, bant, version that works in a couple copies of Linvala, Shield of Sea Gate and sometimes 1 or 2 Glasspool Mimic
in the main, and also opens up the sideboard to things like Dovin's Veto and Mystical Dispute. I noticed those bant lists when looking at tournament finishes in actual Pioneer but have yet to try an Explorer version out on Arena.
The Book of Exalted Deeds is something I'm just starting to try out. I know many other players have used it with a lot of success, and as I just opened one I decided to give it a go. Angelic Ascension is kind of a pet card for me, but it actually does work very well in the deck. Giving your opponent a 4/4 angel is a great tradeoff for exiling a problematic creature or planeswalker, as if you have ANY type of board presence you can just blank it quite easily. I mostly use it on my own creatures, either on a key angel in response to removal or just to turn a mid/late-game 1/1 into something like a 12/12 angel that also gains you 12-16 life and can then cause Resplendent Angel to spawn a new, even larger, angel and gain you even more life in the process.... It's a great 'trick'. ;)
When playing the deck, remember that each Resplendent Angel can trigger at the end of EVERY turn, not just at the end of YOUR turns (pretty crazy with things like CoCo and AA, as well as the Angel's own lifelink ability). Something often overlooked.
