1. Wade Boggs, BOS: 59.8 bWAR/150 OPS+/993 RC
This is a close one, but Boggs just ekes out Schmidt for the honor of best third baseman of the 1980s. This is in large part due to the fact that Boggs didn't actually make his major league debut until 1982, giving everybody else a two-year head start. In spite of this disadvantage, Boggs managed to equal or surpass his compatriots in many stats, of both the counting and rate varieties. He created just as many runs as Schmidt, beat him in WAR (probably due to superior defense), and made up for his home run deficit by being one of the best players of all-time when it came to getting on base. His .443 OBP from 1982 to 1989 is stunning, nearly 60 points higher than Schmidt's OBP for the decade. Even during his monster seasons--more on that in a second--Schmidt wasn't able to equal Boggs's ability to get on base. Heck, Boggs led the American League in batting average five years out of six (from 1983 to 1988) and OBP six out of seven. Boggs is a hall of famer, so it can't really be argued that he flew under the radar, but his semi-tumultuous tenure in Boston and his late-years slump have contributed to him becoming one of the easiest-to-forget superstars of the past few decades. A reappraisal of the start of his career shows just how unwarranted this is.
While Boggs was the 1980s king of getting on base (at least among third basemen), Schmidt was the undisputed best power hitter. He led the National League in home runs five times in the 1980s--he hit the most home runs of any player in either league during the decade--winning the MVP in three of those years. His performance tailed off in 1988 and 1989, his last two seasons in the MLB, but he hardly deserves to be punished for that. Much like with Boggs, it's quite impressive that, in spite of two seasons of reduced (or, in Boggs's case, zero) productivity he was still able to put up such great numbers. He even won six Gold Glove awards, and while those awards are pretty meaningless he did manage to play pretty good defense (41 fielding runs is good for sixth among third basemen in the 1980s).
3. George Brett, KC: 47.5 bWAR/150 OPS+/970 RC
Brett is the final member of the trio of truly great third basemen who peaked in the 1980s (an argument could be made to include Molitor among this group's ranks; more on that soon). Although his numbers don't quite stack up to Boggs or Schmidt, Brett is well-remembered for a reason: the man could hit. His OBP for the decade hovered just below .400 and, while he wasn't as prolific of a power hitter as Schmidt, Brett led the American League in slugging percentage (and OPS) three times in the decade.
4. Paul Molitor, MIL: 38.1 bWAR/124 OPS+/809 RC
Before he was a designated hitting superstar for the Brewers and Blue Jays, Molitor was a surprisingly (to me, at least) fast third baseman. His hitting production was a step below numbers 1-3 on this list, but he was still head and shoulders above the rest of the field. He didn't have any seasons that really jump out for their greatness, but he was very solid from the beginning to the end of the decade, with four seasons of a WAR above 5. He may not have as eye-popping numbers as Boggs, Schmidt, or even Brett, but his inclusion in the Hall of Fame is just as warranted.
5. Buddy Bell, TEX/CIN: 34.9 bWAR/113 OPS+/81 Rfield
Bell manned the hot corner in the 1980s for Texas, Cincinnati, and Houston (albeit briefly) with a deft glove and a strong bat. His slightly lower OPS+ is more than made up for by his fielding, for which he earned 81 fielding runs (the highest of any third baseman in the decade). While Bell's production tapered off at the end of the decade, he accumulated 29 bWAR from 1980 to 1984 with peripheral numbers that, had he kept it up, would have made him a contender for #4 on this list. Still, Bell was quietly one of the great players of the early 1980s, and is a clear #5 on this list.
6. Carney Lansford, OAK/BOS: 30.7 bWAR/116 OPS+/738 RC
Lansford couldn't field a lick, but his bat more than made up for that deficiency. With numbers very similar to Bell's, Lansford loses out due to his seemingly leaden glove (-33 fielding runs), though he did benefit from keeping up his production for the whole of the decade. He didn't hit for power but he excelled at getting on base (getting over a .350 OBP five times), something that allowed him to be quite the productive hitter.
7. Tim Wallach, MON: 32.0 bWAR/108 OPS+/80 Rfield
Similar to Bell, Wallach's slightly lower hitting numbers are balanced out by his superior fielding. Unlike Bell, however, Wallach's hitting wasn't that strong. A decade OBP of just .319 prevented Wallach from being much more than a good fielding third baseman, though he did show various signs of power--he hit 162 home runs for the decade and twice led the league in doubles.
8. Doug DeCinces, CAL/BAL: 25.3 bWAR/115 OPS+/548 RC
Now we're at the part of the list that's decidedly non-elite. DeCinces was a fine player, but didn't do much in the decade, apart from a 1982 that saw him OPS .916, rack up 7.6 WAR, and earn third place in the AL MVP voting. Aside from that season, he hit reasonably well and fielded his position competently as well. And there's nothing wrong with that.
9. Howard Johnson, NYM/DET: 18.7 bWAR/126 OPS+/470 RC
Johnson entered the league in 1982, so his counting stats suffer from two years of inactivity (this is the main problem with lists constrained by a fixed period of time). His rate stats, however, were pretty great, at least when it came to hitting. He's actually pretty similar to Bob Horner, the third baseman for the Braves and the Cardinals in the decade, but I'm inclined to favor Johnson for having the decade begin his career, rather than end it. Anyway, his 1987 and 1989 seasons (4.3 and 6.9 bWAR, respectively) were a big part of the Mets' late-decade runs, even if only 1986 ended in a World Series crown...
10. Gary Gaetti, MIN: 24.3 bWAR/103 OPS+/63 Rfield
Gaetti showed little pop with his bat, but he played a competent third base for the Twins for basically all of the 1980s. He managed to hit 185 home runs, but his .311 OBP is subpar by almost any standard. Most of his WAR was accumulated through his glove, but with a slightly more patient and competent bat he could have moved further up this list. Still, though, Gaetti had a nice career with a good beginning in the 1980s.
Runners-up: Ron Cey, LAD/CHC; Toby Harrah, CLE/NYY/TEX; Bob Horner, ATL/STL.