Well don't hate me because i have it in my backpack. This is my first suggestion and i hope you'll respect it.

Roboactive ones are priced at 4.3 on the unusual pricelist. Antifreeze it's actually the same effect with a different colour.

Both roboactive and antifreeze are high tier effects even if they are not 1st gens.

Proof:

Roboactive ones ---> http://www.tf2outpost.com/trade/24302049 and http://www.tf2outpost.com/trade/24266885

I bought the Antifreeze one for 3 buds pure:

--->http://www.tf2outpost.com/trade/24238031

And it has been sold for around 5 buds in unusuals is the past

Well this is all, if you need anything let me know in the comments!