You obviously like girls' names ending in N.
You might want to consider Alison, Ann, Anwen, Tegan, Carmen, Eden, Erin, Helen, Lauren, Vivian, Catherine, Gillian, Rowan, Susan, Jasmine, and Evelyn too.
They are all really popular if you factor in the fact Ellen is likely to be Ellie and Lilian is likely to be Lily.
I'd say that Megan peaked in the 1990s and while it is well-used, I think that fairly recent burst of popularity means it's not going to be seen as timeless - it's one of these names that perhaps comes in and out of fashion. I think it's still a decent choice and I like the nickname Meg.
I'd say Emily is most consistently popular name of the lot and so, for that reason, probably the most timeless. There are plenty of namesakes but, in spite of that, I do find it a little girlish. It's the kind of name you really want if you're a princess, girly girl but not the sort if you're a tomboy. Emma sounds fresher and I think it's more versatile and mature-sounding.
Lilian is in the vein of these old fashioned vintage names coming back into style. It's certainly a classic girls' name but I'm not sure if it's timeless. Since Lilys are everywhere, it's not one I'd pick - she'll just end up as Lily too. In any case, I'm never sure whether to spell it with 2 or 3 Ls (Lilian or Lillian).
Ellen is nice but lots of mums and dads are turning to it because they want an Ellie and Ella and Eleanor are a bit overdone. In my opinion, Helen or Helena would be a better pick - just off the beaten track so sounds a lot fresher. There's also the French Elodie to consider which combines the rhythm Emily with the El- sound of Elodie.
All your choices are really sensible but very much on trend so you might get a lot of replies suggesting that they are 'safe' or 'boring'. Personally, I'd rather a 'safe' name than an outlandish one but you might want to think beyond the top 25 or so (as I believe all 4 of your choices or their derivatives rank up there). Very, very few of the names that popular currently can be described as timeless so, if that's something you're after, dig a bit deeper. If that's not an issue, pick the one of your 4 you like the best and your daughter will have a very pretty name.