According to a poll run here at Legion World some time ago, the most popular Legionnaire is Invisible Kid. I guess brains, diverse skills, subtle wit and sexual ambiguity go a long way.
The list went as follows:
1st) Invisible Kid I (Lyle Norg)
2nd) Brainiac Five (Querl Dox)
3rd) Element Lad/Alchemist (Jan Arrah)
4th) Timber Wolf (Brin Londo)
Equal 5th) Dream Girl/Dreamer (Nura Nal); XS (Jenni Ognats)
Equal 6th) Cosmic Boy (Rokk Krinn); Duo Damsel/Triplicate Girl/Triad (Luornu Durgo); Gates (Ti'julk Mr'asz); Star Boy (Thom Kallor)
Equal 7th) Apparition/Phantom Girl/Phase (Tinya Wazzo); Ferro [Lad] (Andrew Nolan); Saturn Girl (Imra Ardeen); Shadow Lass/Umbra (Tasmia Mallor); Ultra Boy (Jo Nah); Valor/Mon-El/M'Onel (Lar Gand)
8th) Dawnstar
Equal 9th) Chemical King (Condo Arlik); Karate Kid (Val Armorr); Sensor Girl (Projectra)
10th) White Witch (Mysa Nal)
Equal 11th) Polar Boy (Brek Bannin); Wildfire (Drake Burroughs)
Equal 12th) Chameleon [Boy] (Reep Daggle); Lightning Lad/Live Wire (Garth Ranzz); Shikari
Equal 13th) Kinetix (Zoe Saugin); Shrinking Violet (Salu Digby); Sun Boy/Inferno (Dirk Morgna)
Equal 14th) Andromeda (Laurel Gand); Colossal Boy/Leviathan (Gim Allon); Matter-Eater Lad (Tenzil Kem); Supergirl (Kara Zor-El)
Equal 15th) Kid Quantum II (Jazmin Cullen); Light(ning) Lass/Gossamer/Spark (Ayla Ranzz); Quislet; Tellus (Ganglios)
Equal 16th) Blok; Bouncing Boy (Chuck Taine); Magno (Dyrk Magz); Monstress (Candi Pyponte-Le Parc III)
Equal 17th) Dragonmage (Xao Jin); Infectious Lass (Drura Sepht); Invisible Kid II (Jacques Foccart); Spider Girl (Sussa Paka); Superboy (Kal-El)
Equal 18th) Sensor (Jeka Wynzor); Tyroc (Troy Stewart)
Equal 19th) Computo (Danielle Foccart); Impulse (Kent Shakespeare); Thunder (CeCe Beck)
Equal 20th) Catspaw (April Dumaka); Gear (I.Z.O.R.); Kono (Brita An'nan); Magnetic Kid II (Pol Krinn); Neon (Celeste McCauley); Reflex (Devlin O'Ryan)
Wooden Spoon) Kid Quantum I (James Cullen)
I wonder how the latest Legion series may affect the popularity of various Legionnaires.
PS Not sure who ran the original poll - maybe it's still floating about somewhere?