How underleveled have you been when defeating the E4?

I just defeated the E4 in VC Crystal, and my lowest leveled Pok?mon was 37 and my highest 41 upon defeating Lance. Lance's Pok?mon range between 44-50. I'd say I was only slightly underleveled.

Have you ever been significantly underleveled, and still won?

Well, i have beaten blue with my level 59-60 team in pok?mon blue, and i still won, but in pok?mon red i needed to grind my team atleast at level 65.
My lowest level were 58 and my highest is 59-60 in pok?mon blue as i said before.

I don't like grinding in these games, so usually one member of my team will be level 50, with the rest being low level 40's.

I didn't grind to beat Red either in Silver, but still managed to beat him with a team of low level 50's.

I think the lowest levels I got was when I beat the E4 in Blue with three Pokemon around level 45. Technically I had four, however I never actually needed to use the fourth one.

I also beat Red in Gold with Pokemon at level ~50, though that really was a pain in the ass to pull of.

In White I also had a team of around level 50 mons when I beat the league. I had to get creative in order to win the final battle.

Never happened. I'm usually way over-leveled with my play style.

In Generation I, I would try to get my team to at least Level 55 before I'd challenge the Elite Four. By the time I'd get through past Lance, I'd still be at least 5 levels behind Champion Blue's Pokemon, and 10 levels behind his fully-evolved starter, which wasn't a problem at all back then.

Given the increased difficulty of the battles in Gold/Silver, I'd grind my team members to level 50 before I'd enter the first battle, though I probably would have gotten away with level 45-47 upon starting with Will.

In Alpha Sapphire, my team was around level 60 before starting the Elite Four, otherwise I wouldn't stand a chance against Steven's Mega Metagross.

So no, I haven't been underleveled for the League post-Gen I.

I honestly always gotten by with a team with levels of half of them in early 50s and the other half around 47-49. I don't know so much about the newer games though, I haven't played past Gen5

I was probably around the 40s in Black, but I don't remember well. That was a long time ago... Though I recently finished a Soul Silver run with 46-48s.

I remember using a Starmie, which was a pain without Psychic-Type moves -.-'

Oh yeah, my team in LG was in the late 50s as well, so I guess that's it

well i wouldnt say won, so to say, but my first ever playthrough of a pokemon game, pokemon black, my team comp was terrible, and my lowest level pokemon was simipour and i dont know if it was lvl 14 or 24, i lost spectacularly

Playing Blue on the Virtual Console, my highest leveled was a lvl 45 Charizard, and lowest a 36 Victreebel, as of E4 and Champion.

I have OCD about levelling up, so mine are always 50+ I try not to let them get over level 60 though, that kinda takes the fun out of it.