Depends on several things:
1. Wood type
2. Splitting technique
3. What you can find in your area
4. How much work you're willing to put into it
Which means -- pine's a different deal than splitting maple and you'd want a different axe. If you've split with a maul you might have to learn to split with an axe (that's my thing right now, way more experience with an 8-pound maul).
If you have a timber supply company in your area that carries some good axes, or a good hardware store, you'll have more immediate choices that you can actually pick up and see how they feel. Little hard to do that online. In new axes, most guys will tell you that Collins today ain't the Collins of yesterday, and in my opinion they'd be right. Still and all if you buy a brand you recognize from a US maker and stay away from axes made in Mexico or China you'll be getting fair quality at the very least.
If you have time to look for axes and don't care when the wood gets knocked up, then check into Council Tool axes. Also look at the Swedes: Granfors Bruk, Hults Bruk and Wetterlings are all major brands. Some like 'em, some don't. Granfors and Wetterlings are owned by the same company now but they say they're keeping the brands separate. Hults Bruk imported a lot of axes into Canada and the US. They're OK axes (I have a 3.5# single-bit) and very good steel but the profile is not as "cheeky" as an older American axe.
If you want a good used axe then look at auctions, tool sales, secondhand stores and flea markets. Plan to buy an axe just for the head (if it still has a handle). Then buy a good handle and re-hang it for your style. Good handle makers is like good axes -- if I don't make my own from wood I've harvested I generally buy a Link handle and rework that. This is where the "how much time are you willing to spend" comes in.
Some prefer to split with a single-bit, others like the straight handle and balance of a nice old double-bit that's been sharpened enough to have a cheeky taper to it. Start with one axe, then get another one and see if you like the difference. Soon enough you'll have a wall full of axes in the shed and still be on the lookout for whatever you think is the perfect axe.
Cheers, eh!
Skiv