I've got nearly every colour Tamagotchi (bar the TMGC+C and, for now, the Anniversary M!X, which I'll be getting for Christmas). I felt like listing my favourite bits about each colour Tama and then which one was my favourite and why.
iD - This is adorable in it's simplicity. For me, out of the colours I've got, this is the most like a vintage Tama, just in colour! There's not a huge amount to do, and I don't think there's anywhere to unlock particularly, but it's super cute and I actually love that there's only a few characters on it. I also have the iD Station, which for some reason makes the iD all the more enjoyable (even though I have an IrDA dongle and can access downloads anyway); I really like connecting it for the games and the photo booth/cinema is fun! I initially wasn't interested in getting an iD but then I saw a gorgeous custom one and I just had to have it; no regrets either! I love running it alongside more 'complex' Tamas, such as the P's. Speaking of which...
P's - I know I said I like the iD for it's simplicity, but I also love how much there is to do on a P's! The Pierces are far more fun than they should be, too. I used to enjoy researching a pierce (what characters, items, games each one had) and then going on a hunt to find it at a reasonable price. The P's also seems to have loads of characters that I haven't really seen on other models, including Chokomakatchi! She's my favourite, as she reminds me of my real life pet (a tan and white guinea pig).
4U+ - Again, there's a simplicity to this one that's nice. I also like how it's not too needy; twice I forgot to pause it before I went to work and both times my Tama survived (they were not very well, but they were still there!) which I can't ever imagine happening on my other models! I also feel like the graphics are a bit more sophisticated. The sprites are probably exactly the same, but they just feel so much crisper on the 4U+ and the backgrounds/destinations seem more detailed. I also enjoy some of the little features like the farm and the NFC bump games; they're nothing big but they're sweet. I also adore the animations; they seem more complex than a lot of the previous models (e.g., the geie collapsing the hosue, or going back in time with the space ship). Even my boyfriend has said he thinks the animations are best on the 4U+.
M!X - I have both the Spacey and the Melody versions and, I'll admit, I've not played them as much I would have liked as I haven't unlocked Mermaid Palace/Gozaru Village (I raised characters on both before I realised I'd need to reset and I just can't bring myself to lose one of them) and therefore feel like I'm missing quite a few characters. So they're on pause until I get my Anniversary M!X (and then I'll probably have the same problem there and not be able to unlock everything!) That said, the M!X is, undoubtedly, one of the most unique Tamagotchis going. I don't know if I like it better or less than the 'standard' game play, but it is a refreshing change. I like that each generation is a complete surprise.The graphic details are also amazing, what with there being no on screen icon bars, so everything is bigger. I do love having large Tamas on screen and, much like the 4U+, the animations are great! One time I tried to take my Tama out to the restaurant while he was dirty and he grabbed the mirror, looked at himself in horror and did a running jump into the bath! It was so unexpected and made me laugh.
Which brings me to;
iD L (E and 15th Anni.) - I have two iD Ls, one English version and one 15th Anniversary version. The iD L E was my very first ever colour Tamagotchi and I ran it for two years without any friends. I can't really say why the iD L is my favourite, it just is. It's got loads of content to unlock and a whole host of adorable characters (even if I am having difficulty unlocking them all) but also has a few minor features that I just love. TamaPets are one (I know they're on the P's too, but to a much lesser extent). I also love planting seeds in the garden to see what I'll grow; particularly when they're downloaded seeds! There's also the option to go visit the houses of previous Tamagotchis for dinner or a snack, which is so much cuter than just meeting up with them at a restaurant. The fact it's in English probably helps, as I do feel I am a little more invested as I can actually understand it! The 15th Anniversary version is also adorable; I deliberately got it as I wanted some of the old vintage characters and am looking forward to running Rednosetchi for Christmas! I should also add that both models have my all time favourite in built games; Hamburger and Run & Jump for the iD L E and a sliding puzzle and a maze game for the 15th Anniversary version. The maze game is insanely difficult.
I love all of my colour Tamas, but the iD L is by far my favourite one to run. ^_^