This is one of my favourite topics mainly because I'm based in England. I will be slightly biased here with a slight caveat at the end...
Ayia Napa
By far my favourite, and I have had the most success here. You can have the beach holiday or the party holiday. Its a great mix of many nationalities, expect to meet more Brits and Russians. 15 in 4 holidays. I go here on my own in Aug/Sep. Best time to go too.
Ibiza
Slightly overrated in my opinion and overly priced for what you get. I genuinely think girls go here to just party, and the lay stats here for me have been terrible 4 in 2 holidays.
Kavos
I'm returning here tomorrow! Been here before and I would say its more easier than Ayia Napa, mainly Brits with some mix of Eastern European. 3 in 1 holiday.
Magaluf
Avoid. First time I went here was in 2012 (a long time i know), but here in England we call it Shag-aluf... we were there for 4 days and it was full of guys from wall to wall. 0 in 1 holiday.
Marbella
Classy version of Ibiza, guaranteed to meet 9s and good weather, admittedly your wallet also needs to speak a good game here. 0 in 1 holiday.
Sunny Beach
I've heard mixed reviews about this pace, its not in the EU so it will be hilariously cheap, but you may run into trouble.
With some of the locals perhaps, I would like to try this one out.
Other places
Warsaw
First place I've ever been to and had girls buying me drinks all night wanting to get to know me, this is no exaggeration and the women are just so beautiful. I didn't even need to game these girls, it was just like a walk in the park. 3 in 3 days.
Now this leads on nicely to my caveat from earlier. I would say I'm a solid 8, and I'm doing well in life. I have been gaming chicks for a few years now with relative success and i can honestly say Warsaw opened my eyes to something I hadn't considered. I am also black so I stood out a lot in Warsaw and initially I thought it was a bad thing but my experience there made me venture to other Eastern European countries and the same thing happened. Riga, Vilnuis, Tallinn. I've met more 8s and 9s there than I have in England.
I would say for what its worth, go to places where you stand out more.