Cross-gen evolutions are one of my favorite things whenever we get a new generation, and it's something I've missed since Gen4 (no I don't count publicity stunt Sylveon). So instead of me stating my favorites (which I will do) this will just become a rant about cross-gen evolutions because I'm bored.
1) Eevee is a cool concept, but I don't really like it or it's evolutions. I'd have loved it a whole lot more if it actually got a new evolution every new region that was discovered. I get it would kind of lose it's feel after a while, but it would still be a really cool concept and something to look forward to every new generation.
2) I really like the concept of introducing Tyrogue.
3) Best thing about cross gen evos is giving life to older pokemon that had died. The main reason I love Gen4 is because it gave life back to older Pokemon that were basically useless, and yes whilst most of them still remain useless and I don't particularly like them (lickylicky and tangrowth) I still love them in a different way for giving a new light to some old pokemon.
4) I do get annoyed when people say stuff like "how can machoke be female??" etc so I get I am a little hypocritical in saying this, but I wish Gallade/Frosslass weren't cross-gens and were instead introduced in Gen 3 as gender based evos. It would make their evolution lines make a lot more sense, if female Ralts became Gardevoir/Gallade respectively, and the same for Snowrunt with Frosslass/Glailie. I'm not saying I have an issue when I see a male gardevoir, or a female glalile, but it just seems odd to me that gallade can only be male, and frosslass can only be female, yet their counterparts can be either gender. would make a lot more sense, and make them feel more counterparty if they were gender exclusives.
5) Best cross gens are Horsea line, Snorlax line, Poyrgon line,and Yanma line. Swinub line and slowpoke also flow in and out of here depending on how I feel
6) Togepi and Chansey line are in GOD TIER best cross gen evolutions.