Wow, that's a loaded question Bart...
Each manufacturer has value lines and high end lines. RJS has Grand Cru, Vino del Vida, and Heritage Estates as their value line. Winexpert has Vintners Reserve, Mosti has Vinifera Noble and Domain Traditione, and Cellar Craft has Stirling and Classique collections as their value lineups. Each of these produce an inexpensive wine. In each of these lineups you will find at least one hidden gem that makes you say "I made this from a cheap kit? It tastes like a premium kit!"
Now the premium kits is where all the competition is. In the RJS world, the kits are now in this order of quality: En Primeur, Cellar Classic Winery, Cru Select/ Cellar Classic, Premiere Cru. In Winexpert, they are Selection Estate Crushendo, Selection Estate, Selection. In Mosti Mondiale, they are Meglioli, Masters Edition, Alljuice, renaissance. In Cellar Craft, they are Showcase Collection and Premium Collection.
Now comes opinion: In terms of price for performance, I think RJS makes the best products. The EP lineup is amazing as is quite a lot of the cellar classic series both winery and non winery. It is closely followed by Mosti, which has some AMAZING quality wines once aged in all lineups! Next is Cellar Craft for certain wines, Amarone, Rosso Fortissimo, and Chateau du Pays really stand out as winners! In the Winexpert line, only the crushendos really stand out to me, and they are way more expensive than all the others listed, so I list them last. Again, that's only my opinion. In short, I prefer RJS products because they offer the best and richest taste for the least price where I live.