For the record, at C2 Barbara's ring gives your active character a 15% hydro boost. Unfortuanetly this pretty much only matters for Childefags since Xingqiu is off the field most of the time (still a nice boost to your rainshield attacks though, especially if you have the 2 constellations that buff it). However, if you have R5 Thrilling Tales on her (like you should) it will "snapshot" and apply the attack buff throughout Xingqiu's burst for the 10s buff duration even if he's off field. She was the first character I C6'd, for some reason the game just loved handing me Barbara's in the first month.
Chongyun is great but niche. Good in Xiangling, Hu Tao, and cryo dps comps (so basically just Kaeya and Rosaria for now). He falls off super hard as an actual DPS unit but I guess if you're a hardcore chongfag you can probably hyper invest in him and get some decent damage. Though you can say that about every character.
Other 4* DPS though - Kaeya, Noelle (already talked about)
Razor - pretty much the goto for a physical damage user. Would recommend always pairing Kaeya with him for constant SC uptime. Gets fucked on by Ruin enemies due to high phys resist, but is great vs fatui/human enemies
Ning - Not everyones cup of tea, but a hell of a unit once you get her built. If you happen to get a 5* catalyst from standard and don't have Klee you might as well build her. Godtier synergy with Zhongli. Good with Noelle too.
Xiangling - This bitch has a bajillion different comps she can use, and once you get a better pyro dps like Hu Tao/Diluc you can easily change her to a support. My personal favorite is XQ+Chong+Xiangling+flexible slot (anemo support, pyro support, Diona, Barbara)
If you want info on units and builds in general, I recommend checking these links out:
/gig/'s character build mini-guides - Decent if you cant find anything else for certain characters. Not very in depth, its one of those guides that should moreso be taken as a suggestion
A more in-depth guide on various 4* units (Fischl, Ning, Sucrose, Xingqiu, Beidou, Xiangling Xinyan) - Personally used this to help build my XQ and Ning, also shows you various comps you can use them in
keqingmains.com - More in depth guides on an assortment of characters. Definitely check it out if you want to build Beidou or Keqing.