That question is a bit like asking "what's the best type of car".

It's not possible to answer the question without knowing your requirements and the type of connection you're after.

Sorry bit vague, would be for standard adsl  residential connection, for house with desktop computer in basement, streaming to upstairs apple tv, 2 ipads and mobile phones using the wireless network. cheers

Bigpipe - their modem is not "free", but it is cheap and good:

Bigpipe now offering $99 HG659 modem

This modem/router handles ADSL, VDSL, and fibre, and has wireless ac.

No contracts, no termination fees.

EDIT  There is no such thing as a "free" modem - most ISPs tie you to a contract, and factor in the cost of a cheap and nasty modem as part of their monthly fee.

 I cant tell you the best modem,  but I can help you avoid some of the terrible ones

1) Slingshot provide an absolute piece of garbage. don't use their modem

2) Orcon don't give you a modem. They simply "loan" one to you.  This means if/when you decide to leave, you have to send it back or they bill you several hundred dollars.  There are many stories of modems mysteriously going missing when returned to Orcon and people being billed for them.  Avoid this problem by simply not signing up to them.

Important question:  what modem do you currently have with Spark?  If you are having issue it's quite possible they ae nothing to do with the modem and more to do with your setup, especially if it's wifi issues.

As mentioned above one of the best all round devices at is the HG659 used by Spark.

It's hard to know your exact environment but expecting a single AP/router to cover a 2 story house with WiFi isn't a realistic expectation. A router and a 2nd standalone AP hooked via Ethernet cable or Ethernet over power would be your best approach.

IMO 2degrees currently have the best modem, - Fritzbox 7340 which is miles better than any other modem I've got for free, has VPN, AC wireless, easy bridging, VOIP, NAS and a great user interface.

Best of all - all the features WORK (its really common for a certain feature not to work properly or well on other modems) - and they work well.

I've owned 4 fritzbox modems before and swear by them now - funny name, but very solid product, I like them even better than DrayTek.