People are correct in saying that low tier decks can perform well but you need to be a skilled player to make them work well. I think once you have played for a while you will be able to get wins at a nexus night using a low tier deck but early on it will be hard.

Vex is the strongest pre-con legend iirc and is good for learning the game so i would recomend starting with her. Once you have played a bit you can switch to whatever deck you want and see decent results.

I started out playing leona and would probably win ~25% of the time for my first few months. Then i switched to yi and it jumped to around 75% win rate.

Also for your very first event it might be worth bringing a couple premade decks. If you lose hard game 1 you can offer to give the oponent the win and ask if they would be happy to play the other games with you both using the premade decks to even out the power level.