I used your question (basically) to find dozens of results, (actually just over 3 million results) and I entered at "Best recommended paid win 8.1 compatible Virus programs" and Free as well as Paid versions were listed.
What options are offered by your ISP, and are there any catches involved.
Often 2 or 3 year subscriptions are asked for (not always free after the first 12 months).
These, below, are Alphabetically listed only, while it seems that they were using Microsoft Windows Defender 4.3 as a baseline for comparison. This is from av-test.org, and some may prefer others to compare programs
Titled How We Interpret Antivirus Lab Tests there is a large ad for Bitdefender beside the ratings (yes they also sell it).
While Bitdefender and Kaspersky are listed at the top, while they were using Microsoft Windows Defender 4.3 as a baseline for comparison that will not use the same detection / removal methods. This is from av-test.org, and some may prefer others to compare programs
Virus protection programs for Win 8.1 <= A few basic test results for you to go over.
AV-TEST Reveals the Best Anti-Virus Solutions for Windows 8.1 A Commercial view from Softpedia
Perfect? Nope - Read the above along with This example of reporting and discussion from Wikipedia -
Some vendors don't approve of certain tests. Webroot's style of defense against brand-new threats isn't compatible with some tests. Symantec has stated that static tests have little value and that only whole-product dynamic tests provide useful information. While some don't choose to participate at all, or participate in so few tests that no overall rating is possible.
Webroot Reviews to see that no test is even "remotely good" AV-Test reported in their January/February 2013 report that Webroot SecureAnywhere antimalware engine was 99.8% effective against malware and 100% effective against Zero-day threats. Detection of widespread and prevalent malware discovered in the last 4 weeks was 100%(the AV-TEST reference set, with an industry average 99%. Protection against 0-day malware attacks, inclusive of web and e-mail threats was rated at 95% (January) and 97% (February) in the AV-TEST Real-World Testing.
Webroot had the worst results out of 20 products tested by AV-Comparatives in the September 2012 File Detection Test of Malicious Software, both in terms of malware detection rates and false alarms. Webroot detected less than 80 percent of viral samples, much worse than the 94.4 percent rate of the second lowest detecting product. Among clean files, Webroot inappropriately flagged 210 of them, raising as many false-positives as the other 19 products tested combined. According to AV-comparatives, the "results and misses have been confirmed with several tests and also by the vendor".
AV-Comparatives gave nine vendors, including Webroot, its highest award in its July 2012 Anti-Phishing Test, in which Webroot took 9th place among 18 products tested in regards to the blocking of phishing websites. In AV-Comparative's Performance Test for October 2012, Webroot had the best PCMark score, having the lightest impact out of 20 products tested on a Windows 7 64-bit machine.
A paid / Pro Version of Malwarebytes Anti-Malware will be a lot better than just the Free (clean-up) version that offers no Real-time protection, and is well above SAS (my opinion).