If you're playing on an actual 3DS, there is one advantage to starting with this game (or at least owning it)! There are three Yo-kai you can only get in Yo-kai Watch 2 by having Yo-kai Cam pictures from the first game - Buchinyan, Komasan and Noko. If you want any of those three in YKW2, then having YKW1 is a must.
As you may have noticed, this game has three different versions (and is the only numbered entry to have separate versions outside of Japan!). So which one do you play? If you're only able to get one, I'd recommend Psychic Specters, as it has more post-game content and some of the best sidequests in the series. However, if you can get multiple, starting with either Bony Spirits or Fleshy Souls isn't a bad move either. There are certain Yo-kai, including a befriendable form of a Boss Yo-kai, exclusive to each version that can't be obtained in Psychic Specters. Plus, you can transfer save data from Bony Spirits and Fleshy Souls to Psychic Specters if you're playing digitally! (Which nowadays would either require you bought the game years ago or you're playing on custom firmware.) So really, any entry is fine!
There are some sidequests in YKW2 (two of which are exclusive to Psychic Specters) that I would highly recommend you play before moving onto the other games. These three are important for lore reasons, so make sure to play/watch them! They are:
To access the first one, you must first beat the sidequest "Ghost Club at School", which should unlock during the story and can be started in Breezy Hills. The second one can be started as soon as you enter post-game and go to sleep once. Disregard the recommended levels, there are no Yo-kai battles involved in any of them. The third one takes a LOT to get to (including playing through the Whistory Lessons), so if getting to it is too hard, then watching a walkthrough of it is fine.
This game may be a spin-off, but it's very much part of the canon continuity. It's an expansion on the gameplay of the Blasters side-mode in YKW2, but if you weren't very good at that side-mode, don't worry! You get AI partners to assist you in this game. If it still isn't your cup of tea even after playing a bit of it, then you can move onto Yo-kai Watch 3 without finishing it. You only really need to get the gist of what the Blasters are about.
As for which version you should get should you get it digitally through one method or another or are stupid rich, my personal judgement method is how much you want to befriend Whisper. Yes, he's befriendable in YKW3 as well, but you can do it sooner in Blasters. If you want him on your team, then you want White Dog Squad.
There's also Blasters T mode, which has the expanded Blasters gameplay from Red Cat Corps/White Dog Squad combined Mystery Dungeon's, well, dungeons. (No, this isn't ENTIRELY a Pokemon comparison! There's also Chocobo Mystery Dungeon!) There's a hell of a lot of content in this side-mode, and there's certain Yo-kai you can only befriend either in the mode itself or by clearing its main campaign. If you want to play it, I'd recommend doing the sidequest that unlocks this side-mode as soon as possible: it should be available right outside Nate's house after upgrading his Yo-kai Watch to Rank D during Chapter 3! Playing it periodically throughout the story will also help lessen the amount of time you'll spend playing through the main campaign. I personally had a team for Blasters T separate from my main team once I entered post-game; it's a good way to use auto-befriended Yo-kai without disrupting the balance of your main team!
(Terror Time gets another change in mechanics, but at least you're not required to wait around for a few minutes before the exit appears again.)
Yo-kai Sangokushi (3DS): It's a turn-based strategy game, and it doesn't have too much bearing on the series continuity, so I'd only really recommend this one if you've played other turn-based strategy games and/or you know Japanese. Due to Usapyon's appearance here as well, it'd probably be best to play this one after Blasters or YKW3. Heck, playing it during YKW3 would be viable too.
Yo-kai Watch Busters 2 (3DS): (Busters is the Japanese name for Blasters, btw) I haven't actually played this one, but apparently it's really buggy and not many people like it as a result. The main antagonists are furries, though, so maybe that'll sway you. Probably play this one after Yo-kai Watch 3.
There's an English translation being worked on which you can find here! I'm not associated with anyone working on it, I just wanted to share the project in case it helps someone. (Don't look at the comments on the thread.)
Yo-kai Watch 4 (Switch & PS4): The gameplay is pretty accessible for such a text-heavy game! The objective arrow is replaced with a cute dog. Story-wise, however, you're either going to want to watch a let's play that translates the dialogue so you have an idea of what's going on or watch the Shadowside anime AND Forever Friends. As the title implies, play this one after Yo-kai Watch 3. I don't know if Busters 2 is referenced at all, but you should be able to play this without playing that.
Yo-kai Academy Y: Wai-Wai Gakuen Seikatsu/Y School Heroes: Bustlin' School Life (Switch and PS4): I haven't played this one either, but that's because I don't have any interest in this section of the Yo-kai Watch continuity. Apparently it's got a social life balancing aspect to it, though...? Probably wouldn't recommend this one if you don't know Japanese because of that. Probably play this one after YKW4.