I have a few answers to this question:

Samantha Saint
Jenna Jameson
Kaylani Lei

My first 2 became available later on - but for me Janine or Nina Hartley in their prime would have been 2- not sure of the 3rd so I’ll say a double with Janine and Julia Ann back in their blondage days - saw them on dance circuit back in early 90s probably

I would say the top three I’d be willing to pay a little extra are.  
Madison Ivy
Capri Cavanni
Gia Dimarco

I met them as Blondage back in the early 90s in S FL, hot as hell, and both were really nice!!

I’ve always been a big fan of the Europeans,  particularly the ones making films around 2000.   3 favorites:
       
    Monica Sweetheart
    Sandra Russo
    Meridian

The IF she was available implies she is no longer available for one reason for another, and the present tense, I take that to mean how she most publically looked.  So in no particular order:

Kendra Lust  
Savanna Samson
Janet Mason

1 Vina Sky
2 Nicole Doshi
3 Jordyn Rain (OF)

I would definitely break the bank for:

Nicole is still available, right?

Alix Lynx
Lisa Ann  
Madison Ivy

Tori Black
Gianna Michael's
Adriana Chechik

Esperanza Gomez
Silvia Saint
Alexa Rae

Top 3 if available  
1. Kayley Gunner  
2. Cabbie Carter  
3. Lana Rhoades

Kimmy Granger
Jessa Rhodes
Eliza Ibarra
Kathryn Mae
Cali Carter

Chole Temple
Racquel Darrian
Blanche Bradburry
Clea Gaultier
Candee Licious
Emma Hix

Stella Cox
Gabbie Carter
Autumn Falls

1. Jean Michaels  
2. Nala Brooks
3. Stacy Cruz
4. Mila Monet  
5. Sisi Rose

I've seen many who are mentioned in the posts of others. These are some that I would have gone out of my way for in their prime, had I known what was possible:

Nicole Doshi is available, see her reviews.

Eliza Ibarra is available but you won't like the rate.

Chloe Temple is available, but the agency is questionable.

Gabbie Carter might be available but it also might be a scam
https://www.theeroticreview.com/reviews/gabbie-carter-2127609453-378190/
https://www.clientsfantasy.com/provider/gabbie-carter-escort/

Kaylani Lei might be available but questionable associates.

Kathryn Mae is also available thru Lourdes, Tryst and Alluringintros.eu

My list of one

Having paid outside of my comfort zone once and being underwhelmed, I can only imagine doing that again with an all-time favorite in her prime, which would be Tori Black.

For me, there are a few. But here’s my top ones

Angela White.

I had kendra Lust back in 2018, Kendra lust was awesome, wish she was still available

Madison ivy
Jessa rhodes
Holly hotwife

I got so excited when Nikki Dial popped up on the Tryst website I practically creamed myself. Alas, while yes she is available, it is only as arm candy. Insert Alanis Morissette’s song Ironic.

Don’t think her looks are what they once were, but she’s one of porns best clits. Very prominent.

Well Janine is on the PornCompanions site.  Not signed up for that site (yet) so I can’t say what the cost is, but…
Of course, she could just be one of those listed, but just as arm candy. Honestly, if I’m paying for candy…. I’m eating it.

I’m a fan of natural breasts, but Shalina has my favorite pair of man mades with the best nipples and her performances never ever disappoint.

I mean, maybe it's a generational thing or something, but I have zero idea how it's not KK on the top of every list. But maybe my tastes are too vanilla.

Emma Hix
Kimmy Granger
Jessa Rhodes

I did pay out of my comfort zone for 4  PS back in 2013/2014

Kimmy Granger
Avy Scott
Jenna Haze
April O’Neil (I know she’s around but I haven’t been able to pull the trigger)
Jenny Blighe
Kourtney Kane
Anissa Kate

Every time she comes out to the East Coast I try to find a way to make it work and never can.

Great question and I would have a list way too long, so breaking it down by category and these would all be during their prime.  
Blonde-Kayden Kross
Dark/brunette-Lanny Barby
Latina-Reena Sky
Asian-Kira Kener
MLF-Nina Hartley

Kira Kener, under the radar one of the hottest ever. But given the tens of thousands who have run through the business over the years, that title can be held by many.