3DS is the most mobile friendly of the current offerings. The clamshell design and small formfactor make it perfect for that quick pick up and play session on the go. The step counter and street/spotpass add a minuscule amount of fun to the act of carrying it around with you everywhere.
Second place is a wash, really. Currently leaning a bit towards the Switch, but it's really hampered by the bigger size and ass-backwards flap on the game card slot. And the fan, which means you can't simply put it into a pocket or lint will gradually invade the system. Nonetheless, still pretty portable, and the sleep/resume function is perfect for portable gaming.
Vita s a Mini Switch, kind of. Still has more games, but at this point in time it's dead, or if we are being diplomatic, near the end of its life. So if you have one, it's great, but whether it's really worth an investment nowadays is debatable.
Mobile is mobile. Loads of different systems, at least two competing platforms, and an insane amount of games. Many (some might even say most) of them are F2P trash though. (disclaimer: F2P doesn't mean a game is inherently bad, but it's far too easy to make a game unfun if it's F2P.) Unless you want to pack in an extra controller, you're forced into touchscreen controls, which may limit your enjoyment - however, you're always carrying a smartphone with you. So this is more of a side console, of sorts. Plenty of disposable fun, some great games, but ultimately kind of a jack of all trades, master of none.