There are 16 undefeated college football teams remaining six weeks into the 2019 season, and all of them have College Football Playoff aspirations. Of course, there is a finite period of time for these teams to not only remain undefeated but also in contention for a national championship -- including this week when we are guaranteed to see an undefeated fall as No. 5 LSU hosts No. 7 Florida.

Some of these unblemished teams are legitimate title threats, while others are pretenders who will see their successes tail off down the stretch of the season. Some of them may not be championship-caliber even if they do run the table. (Just ask UCF how that feels.) Attrition is inevitable in college football, especially with so many of the teams below playing one another as we roll through the second half of the season. (The halfway point is approaching at the end of the week, mind you.)

But how do these undefeated teams stack up right now? Which ones may we see competing for the national title later this season? Let's separate the contenders from the pretenders and rank them top-to-bottom starting with the best teams in the nation. 

1
Alabama
Let's not out-think the room: Alabama is the best team in college football. The combination of quarterback Tua Tagovailoa and track star wide receivers will make other teams score 40 points to even have a chance. Not many teams can do that against that defense.
2
Ohio St.
Quarterback Justin Fields gets all of the publicity, but the real driving force behind the Buckeyes' early-season success is giving up just 3.69 yards per play. Coach Ryan Day has put together the most complete team in college football. We just have to find out the height of Ohio State's ceiling.
3
LSU
Quarterback Joe Burrow and passing game coordinator Joe Brady have successfully implemented a foreign concept in Baton Rouge -- a wide-open passing game. No, the defense hasn't been up to LSU standards, but it's just now figuring out how to operate now that it's on the field for longer periods of time due to the tempo-based offense.
4
Georgia
Georgia answered its offseason questions that centered on wide receivers and generating pressure on defense. But we haven't had to see the offense open it up when it is necessary just yet. That's a double-edged sword. It's likely that quarterback Jake Fromm can light it up with this crew, but we haven't seen it.
5
Oklahoma
Quarterback Jalen Hurts leads the nation in passer rating, the offense has picked up right where it has left off and the defense has taken a small step forward. Is that step due to actual success or subpar competition? That remains to be seen.
6
Clemson
The defending national champions were taken to the brink by North Carolina, and star quarterback Trevor Lawrence hasn't exactly looked crisp. They have the talent on the field and on the staff to get right and take off, but it hasn't happened yet.
7
Wisconsin
Two important points on the Badgers: Star running back Jonathan Taylor is now a threat in the passing game, and quarterback Jack Coan is averaging 8.2 yards per attempt. This is the best Badgers team since Russell Wilson took the snaps in 2011 (and it could actually be better).
8
Penn St.
First-year starting quarterback Sean Clifford leads the Big Ten in passing at 288.6 yards per game, and its defense is giving up just 7.4 points per outing. Ohio State gets all of the pub, but it's about time Penn State starts to get some love.
9
Florida
The Gators defense is nasty. If you don't believe me, just ask Auburn about that. But what happens if the defense has an off day? Can Florida muster up a ground game, and can junior quarterback Kyle Trask -- who took over after Feliepe Franks was injured in September -- keep up in a shootout? That would be a tall order based on what we've seen, tough Trask did lead UF back against Kentucky.
10
Boise St.
The Broncos are the class of the Group of Five, and they have a superstar freshman quarterback in Hank Bachmeier who has been as tough as nails this season. Can they knock on the door of the College Football Playoff, or will they just be a UCF-like afterthought? That might depend on how good that Florida State win looks at the end of the season.
11
Baylor
Big 12 quarterbacks have been the recipients of a ton of attention this year, but none of them have been named Charlie Brewer. All that the Bears quarterback has done is throw 11 touchdowns without a pick. Pair that with a defense that's giving up just 306.2 yards per game, and you have a pretty good team.
12
App. St.
The Mountaineers already notched a Power Five win at North Carolina and lead the Sun Belt in scoring at 47 points per game. Junior running back Darrynton Evans has averaged 7.03 yards per carry, and quarterback Zac Thomas has completed 71.9 percent of his passes.
13
Memphis
Quarterback Brady White is averaging 10.3 yards per attempt, and Kenneth Gainwell has had no issue stepping in as the No. 1 back on the depth chart after posting three straight 100-yard games. But the offense has sputtered at times -- namely against Ole Miss and the first half vs. Navy. Can the Tigers be trusted?
14
SMU
SMU is the feel-good story of the season. Once left for dead by NCAA sanctions, the Mustangs are bowl eligible thanks to a stellar offense led by Texas transfer Shane Buechele. They've scored 37 or more points in every game this season but have given up 27.3 -- eighth in the AAC.
15
Wake Forest
Quarterback Jamie Newman leads the ACC in passing with 304.2 yards per game, and the dynamic receiving duo of Sage Surratt and Scotty Washington is one of the best in the nation. The third down defense is fantastic (25.68 percent), but Wake has struggled to generate negative plays (31 tackles for loss -- 13th in the ACC).
16
Minnesota
Tanner Morgan is third in the Big Ten in passing at 255 yards per game, and running back Rodney Smith is one of three Big Ten players that average more than 100 rushing yards per game, but the offense as a whole is averaging just 3.49 rushing yards per carry. Rowing that boat is a grind.