First decide whether you need two or three prongs. Even if your current adapter is three prongs take a long hard look at the laptop charger market for a replacement non-grounded adapter. You will save weight and a ton of hassle by not needing a three prong cable and/or adapter. We have a USB C charger wiki which includes the Innergie 60C, a 60W two prong tiny charger for USB C and with an optional cable legacy laptops as well.

Also consider whether your chargers have replaceable AC cords, the so called "figure eight" and "mickey mouse" cords are replaceable; they are readily available on eBay for any nation and it might be a better solution than the original cord + a shape adapter.

Now for plug shape adapters. The current best choices are: MOGICS Bagel/Donut because it's a powerstrip/USB charger/adapter combo and still small enough. Also ran: Gocon Alpha because it's flat. The Wonpro Nano would be good because it's two plugs and very small but it is becoming less and less usable in Europe.

In detail:

1. MOGICS has a donut shaped travel power strip, it comes in two variants, one for US plugs only (Donut) one for universal plugs (Bagel). The Donut is pointless, the weight gain is minimal, and it's only marginally thinner, just get a Bagel. Both has two USB ports. Also, their travel adapter while much less universal (US to UK/EU/AU only) is really small, together with the Bagel universal enough and fits inside a grounded Schuko outlet. Bagel weight with case and MA-1 adapter: 6.15 oz. MA-1 alone: 0.92oz. If two USB sockets is enough this is your best choice. Bonus: they have the smallest MicroUSB cord you'll ever see. They licensed their unique shape to the Mara and Masa Kickstarter, once it arrives I will add it to this wiki, until then it's in the USB C wiki.
2. The often mentioned Kikkerland is again a totally pointless product. It's a knockoff of an older version of this Japanese product. Either buy the real one or purchase a Chinese knockoff for even less money. There are many knockoffs on Amazon.com for even more money. Note: the original had some problems with the UK now this has been addressed with an added switch. The knockoffs don't have this switch. Knockoff weight: 1.82oz.
3. The Wonpro Nano was very good but less so now, read on. Yes it's not flat but it's small enough to be pardoned for it especially because you can plug two devices in. It's quite probably a Nissyo NP-10 knockoff but since the Nissyo is unavailable outside of Japan I can't be bothered. I had more success with this in the UK than the one above but that was before they added the switch so they are now probably even. The way the plug unscrews for the EU plugs allows for a very small physical size. For a while this was a favorite but a fatal flaw has been disovered: some grounded Schuko sockets have child-proof socket shutters built in (and this is now widespread) and these screwed out legs can't unlock the shutters. Some countries, including Portugal, Finland, Denmark, Norway and Sweden, require these which causes almost all power strips now to include them further decreasing the utility of this converter.
4. Another useful powerstrip is the Road Warrior 47. Weight: 4.14oz. Especially with this weight the only advantage is being flatter than the Bagel.
5. If you need a three pole travel power strip try the Voltage Valet PS2 Travel Power Strip.
6. Any unwieldy big brick style plug adapter with sliding out plugs is for the kettles. Exception is the Skross World Adapter PRO because it's an absolutely unique three poles adapter (not counting Chinese knockoffs). Yes, Tumi also sells an adapter like that but if you look closely you'll find the Skross logo on it Weight: 6.56oz but if you need to convert something with three prongs you have no choice.
7. Finally, boycott Monster. They are a tax evading patent troll, don't fund them.