Well firstly in the bar situation you need confidence, then a degree of charm and good looks are also a bonus. With those foundations in place the chat up line could be pretty much anything funny / complimentary / cheeky. You get my drift.
When using an online dating site you can read profiles and see photos online. You can get a feel for that person you are about to contact. More importantly you have time to prepare your first icebreaker message to ensure you make that great first impression.
Don't copy and paste a generic message e.g. "I think you look amazing and would like to meet. Click on my profile and message me if interested". This is not going to work.
So what is the best chat up line to use? Well there isn't one magic ice breaker that will work on every person. But the secret to establishing a conversation is to keep it relevant and sincere. Be sincere and ask a question about something that piqued your interest when you viewed their profile. If nothing stirred your interest then don't waste your, or their, time. I mean, seriously, why would you think it cool to contact someone who you find of no interest? Is it because their profile picture was hot? In which case please get in line with all the other optimists hoping for a miracle.
Writing an interesting profile takes time, and can be a little nerve racking. A persons dating profile is posted to help you break the ice. So read what is posted. Try to understand the person behind the words then reply by:
1: Introducing yourself, tell them your name
2: Mention what you find interesting about their profile
3: Ask a relevant question. People love to talk about their interests, so instigate that opportunity.
Your icebreaker message is your opportunity to make a first impression. One of the benefits of using any of the dating121.com sites is they allow you the chance to send free Icebreakers. So what are you waiting for?