I can't speak for gigapets or most other oddpets (non-tamagotchi and usually non-bandai vpets. i.. dont know why they are called that but they are)

howeverrr thats a Tough Question. Normally I'd say mid connection era around v3 or so especially if you are new to the hobby and thankfully! theyve added connection models to their "original" line with a lil mix of content from each one. I find they have a very nice balance of nostalgia/retro vibe + difficulty of care + Stuff To Do. They have waaay more character and minigame options than the p1/p2 v1/v2 (og 90s vs original line rereleases) and not too hard but not too easy (and you can pause if you must). But I've really REALLY been loving the Uni. Its the first tama with wifi connectivity and they have like little team events and QOL patches and even dlc if you can believe it. It also comes with a wristband to wear as a watch (convenient!) and is charged via USB. Certain characters are way tougher to care for than others like noticeably so but its not too bad its one of the ones where you can keep your tama as long as you can keep it alive and ive had 100+ day old tamas before i went and married them off to get a new eggy. HOWEVER it does not have the genetics features that the mix and on (the wonder garden is an on) have what you get is what you get. no color or eye variation like on the paradise even. I've only had the paradise for like half a day but the zoom gimmick is soo cute and it is the cheapest color model out there but if I end up liking it more than the Uni only time will tell.

Oh and I'm not crazy about them but the tamagotchi nano (usually a collab with some other IP, i have the eevee and gudetama ones in my pic) do have a lot of variety but they are so. SO easy and simple youd have to forget it on a counter for DAYS for it to croak (except for the gudetama where it has a gimmick where you collect different death scenes which is a fun twist) they all play pretty similarly but if theres one with characters you really really like its a cute little thing to have on a bag.

I realize i basically suggested like 4 different things and thats not entirely helpful but like different models/eras hit different and im not right now* but i have run multiple tamas at once and its pretty common in the community to do so lol

*my uni needs to charge the battery ran out but i do plan to run both uni and paradise lmao