I think it really all depends on what you have at your disposal and your preference. As each colour tamagotchi has its own flavour of the experience you are going to get
The plus colour, the first one to feature colour is the only colour tamagotchi to have oldies present, and you can even take your oldie to the matchmaker to marry an oldie. This one is one you need to play it slow. There are 7 lucky items that give you happy symbol, you need to get these each generation if you do (it is rather challenging in some seasons) you get to play the cash grab game at gotchi castle. I truly love this version for how detailed the weather function is. It does have a less extended growth chart that people are used to for more recent releases, but this tama has charm and if you take it slow you will unlock more things over time. This one holds a special place in my heart
The ID was the first to introduce tama tomo stamps that are specific for each adult character and to collect all for to collect characters in the book. There are 11 adults in this one, no oldies (so sad) and it's pretty basic in my mind. The photo booth is cute, you get to dress your tama up and take a picture. This one also features downloads which are readily available online, you can have new games, photobooth stuff, meals, snacks, toys and accessories. I find it to be low maintence.
The IDL series is amazing, the princess spacy or the anniversary are my favourite. They are complete and each have their pro's and con's. If you have the capability for downloads well that just opens the doors, you can change up the games and wallpapers and seeds and items and clothes. This tama features planting seeds that will harvest food and toys. We have added clothes so you can dress up your adult. Tama tomo stamps are still a thing, and there are 32 characters to collect (4 special ones you get from connecting with others. either 5 different tamas or 30 connections (where you can only use one)
The P's is by far the most well loved colour tama and has the most added extras that go with it. There are the peirces (which are sold separately and some are sold at what I call ridiculous prices). Like the IDL there is a heap of downloads out for it, in fact more so. There are download collections that are english, there are custom made items/ wallpapers, and so on. And thanks to the Mstr blink team we have the ability to patch it english, download added characters (VDPS) and recently they have been working on complete pierces ( right now there are 2, both are change pierces, so you get the 10 adult transformations, items, meals, snacks that originally came with it and it's all in english). There are again 32 built adults and the 4 connection ones. These ones are a tad trickier to get you need to do a three way connection 5 times to get 2, and you need to connect to 5 different opposite gendered tamas as a teen, so if you are a completionist this is something I feel is important it was the reason that I disliked the P's a lot. But then again if knowing you will never compete your tama tomo book (and building the rocket) then okay the VDP's and all the bonus stuff will keep you busy
Next comes what some refer to as the big flop. The 4U/4U+. I don't dislike it, because there are some serious features that I wish were on others. They are less needy, there is daycare that while they are there their meters will remain as they were so pretty much you can send it to day care and not worry. You potty train, train them to bathe and train them to clean up their room, which I love. I also love that adults learn skills in school and these skills can be used for money making ventures. The games are simple and one can even be regarded as a button mash. This is perfect for busy people. Yes there is a smaller growth chart than expected after the Ps. There is also post adult evolutions so how you care for your adult will transform them which I think is pretty neat. If its a toss up I would go 4U+ there are 12 built in characters, there are a couple new destinations, some other new things including new post adult transformations (which if you count then almost double the growth chart and some aren't all that easy to get)
And next comes the new and exciting and constantly having new models released the Mix. I love the mix. This is a tamagotchi that I can run for months and never have the same adult twice. Sure some may be close but they are not the same, and some of the mixes are adorable (Ever seen a lavender kuchipatchi with melodytchi ears?) growth for the most part is random there are preset babies toddlers and teens and the mix traits are added to that starting with colour, so you really do get to see them grow up. There are two forms or daycare, (each are free and each dont' let the meters drop). I love the fact that there is depth to the room, and tons od random animations. We have returned to the egg design and even the beeps it makes just sound more refined. Granted it's not perfect, when the happy meter is low your tama will have a pouty face and t times this is cute others infuriating The happy meter isn't easy to fill.... like at all you need patience and to alternate from toys, snacks, washing, cleaning house, and visiting others. There is no match maker instead you travel to different locations (which you unlock by various means) and you have to make friends and whoo those of the opposite sex to marry. Each have at least one item they love (which of course makes your relationship grow faster). So out of the many versions which do I suggest? Well you only want one, so I would suggest one in which you don't need any others so I would go with an anniversary mix. You don't need any others to unlock all the locations (the others even sanrio has the mermaid place which requires connections to unlock which really I dislike).
So what should you get?
Well you only want one, so in that case I would suggest one that doesn't require any others to unlock all the content so you have plus colour the ID, the 4U/4U+ (you do need NFC random connections and that can come from any NFC device 3DS random phones etc), and the anniversary mix. These are the ones I would suggest for someone who only wishes for one colour. If you want one that you wont get bored with (seeing the same characters over and over) then I would suggest the mix for the whole mixing traits and such thing.
