Stevens undoubtabely the biggest waste of talent, he's such a talented and fluent player it's a shame he hasn't won the titles his talent deserved. He was such a good player between that 1997-2005 period, probably the best standard of snooker ever, he was always in the latter stages of tournaments but somehow when it really mattered something wasn't there to make the final push, plus as we know when close to the winning line he seems "afraid" to cross it and has bottled many matches, his 2 World Finals when being well in front just an example of that, but there are many many more when he really should have won.
Only 1 ranking title, the 2003 UK Champs is just way too short for a guy of his talent, he should in theory have at least 5 or 6 ranking tournaments already to his name. Still, he did manage to win the Masters in 2000, and sometimes that seems to go under the radar by some.
As for Maguire his case is not as worse as Stevens because he has won more ranking titles and doesn't have that tendency to bottle big matches when being well in front. Often when he loses it's because his opponent got to one frame of winning the match first, not the reverse as in Stevens case.