Information about the devices tested here.
Based on this, our current recommendation is this particular USB, which is the cheapest yet leads to a performance above LENA's: MP power @ Clé USB Numérique Enregistreur Flash Memory Stick Voice Dictaphone Recorder 8Go 8Gb 8 Go
If you want to do even better, you can get this one: Enregistreur Vocal 16 Go, Enregistreur Micro Discret Numérique Activation Vocale, Dictaphone Portable Rechargeable par USB MP3 Player pour Cours, Réunions et Entretiens