There are so many shampoos that are so similar. Really - you can use anything from PBMG and most of them work just fine. Here are some that stand out off the top of my head for various reasons:
All shampoos at dilutions recommended on the bottle...
1. Prima Mystique: Just a crappy soap. Doesn't foam well. Costs a lot. Smells like lavender if I recall, which I don't like. Basically, a crappy soap from a company who makes great stuff.
2. Sonax Gloss: a nice shampoo. Reminds me of hair shampoo. Can't fault this one. Extra points for being German.
3. PBMG soaps: Most are good. None are worth the $$$.
4. Optimum shampoo: Amazing in the fact the suds will literally last in the bucket piled high for days. You can foam up a bucket, leave it there, and suds will be there 2 days later. Would buy this by the gallon when the time comes.
5. CG Honeydew: One of the only soaps I buy by the gallon. Nothing seems to work better in a foam cannon and price isn't bad. Love the smell. Foam seems to cling to paint better than anything else I've tried in a cannon.
6. DP Foam: Not as good as Honeydew. Costs more. Smell is mint which I like. Only get if on BOGO.
7. 1Z Gloss: Great shampoo for 2 reasons. First, it's super concentrated. An ounce per 5 gallons is more than enough. Second, you can see the gloss it adds. Would buy this again. Plus +1 for being German...
8. PBs SSS: Great shampoo. Does everything well. I like it and like supporting PBs.
9. Lusso Oro: Luxury. Luxury. Luxury. Super smelling, rich, foamy shampoo. Cost a lot. If you roll wax & $80+ gallon shampoos - this is for you.
10. P21S: I like this. It feels special, but it's not super foamy. Says it's made of a bunch of natural junk. I use this when the car is only a little bit dirty. I shy away now from saying a soap is "mild" - because I'm not sure how you even determine that.
11. CGs Citrus Red: Use this before polishing, because I've bought into the hype it's a "strong cleaner". Really, when you dilute it an oz per gallon it makes a ton of foam and the solution feels substantial. I also use it 1:30 in a spray bottle to clean wheels. I buy this by the gallon. Not sure why you can't buy it from CGs directly.
12. CG's Gloss Works: the smell is awesome (watermelon) and it's a solid shampoo. Whatever the "gloss" stuff is in it, it inhibits beading - won't buy this again.
13. CGs Citrus Wash n Gloss: Same beading problem as above. It's a pain you have to shake it. After a while it looses its bright yellow color in the bottle and looks like old milk. Having to shake up a gallon of this would be a real PIA. Although, dilution ratio is very good - so it's a bargain.
14. Duragloss: A real good shampoo. Wash solution is very substantial and foamy - but you use a lot (1oz : gallon). Smells like Robitussin with Codeine (cherry) - not a big fan of the smell.
15. Ultima: People seem to love this, I'm not quite sure why. It's OK - not great. Doesn't smell like much. Works fine enough.
16. Dodo's B2BM: Expensive for what you get. Nice packaging. Prolly would only buy this if your using a Dodo wax.
17. Vic's Super Soap: Very concentrated, uniquely smells like apples. Color is clear. Use with wax. Good suds. You should try this is you are into wax.
18. Mothers Wash n Wax: Not bad. Doesn't stand out much. A little weak on suds. The color of snot. Works just fine if your not looking to spend a lot.
19. CG's Mr Pink: Doesn't excel at anything. Reminds me of Maxi Suds just diluted down. Basically, you can do without this one.
20. Sonus: completely unremarkable soap. Skip it. But if can get it for free it will work OK.
21. CarPro HydroFoam: only use this for wheels in a spray bottle, and I like it. Clean and protect in one step. Awesome beading. Decent foaming. Way to expensive to use on paint on a car with LSP. Basically, a great niche soap.
22. Megs OTC "Crystal" shampoo: Skip it. Just a cheap shampoo. You need a ton of it to make even decent foam. It's clear and I don't recall it smelling like anything. Dunno if they even make this anymore. If you have to go OTC - go Mothers.
23. CGs Maxi Suds: Works well in foam cannon and in bucket. Priced right. Good all around soap. May degrade wax. Prolly good choice is you roll a sealant and don't want to spend a lot.
24. Griots car wash: Unremarkable. Doesn't foam well. Nice packaging. You use a lot. Expensive. Skip it and get PoorBoys or Vic's.
This is all I can think of off the top of my head, without going through my collection.