There are as many answers, as there are atoms in the universe. Everyone has a favorite. It comes down to cost among other factors.
You're moving up from a micro .380, so is SMALL a major concern for you? If so, then in the land of reasonably priced guns you've got the Kahr CM9/PM9 which are extremely small, and also Beretta Nano (recently restyled as the APX Carry). Not much larger, but with twice the capacity is the Sig P365. You'll pay $500 for that one but get 10/12 round capacity and very nice sights at a size smaller than most compact 9mms.
Also in teeny-weeny land are the 1911-styled and usually very expensive 9mm's from Kimber, Sig and Springfield. Gorgeous guns, but you're paying for fashion and style. You're required to wear a tuxedo and drink martinis when you carry one. Note: The new Kimber Evo striker fired, but 1911-looking-ish gun, is AWESOME. And you'll pay for that awesomeness.
A notch up from that in size (but barely) is the endless crop of single stack, 6/7/8 capacity guns that include S&W Shield, Walther PPS M2 (those are both very affordable and both outstanding guns), Glock 43, and a host of vendors like Ruger, Taurus, Honor Defense, and too many others to mention.
If you like a wider grip, you can get into the subcompact double stack guns like Glock 26, Walther PPQ Subcompact and Springfield XD Mod.2 Subcompact. All great guns, but the existence of the P365 kind of makes you go "why???" at these guys. Of course, each of these is compatible with 15+ round magazines from their larger siblings, which does make a very good case for a gun you can carry and conceal easily but also do extended range and training sessions with.
Based on my purely subjective criteria:
If absolute smallest size is your goal: Kahr CM9
If capacity is of interest: Sig P365
Price: Shield 2.0 or PPS M2 (you can get other brands for less, but these two are known to be absolutely reliable and have good aftermarket support)
Raw sexiness, and you're willing to sell both kidneys to pay for it: anything from Kimber
Most practical, flexible, all-around platform and willing to move up in width and weight: Glock 26 or PPQ Subcompact