I've been a fan of the HER-2M for years. Its 7/11 speed easily puts it into the rear arc of an opponent, with triple MLs to punch holes and a pair of MGs to refill those holes with crit-seeking lead. The Flamer is useful for anti-infantry work, starting fires (always amusing), or delivering heat to an already overheated target instead of firing one of the MLs. It can run and fire everything except either the Flamer or one ML without building heat, and its armor is pretty close to the limit for its tonnage. I've taken on and taken down some really nasty heavies and assaults with these over the years, and consider it my favorite one-on-one medium duelist, as well being as a great scout-hunter and backstabber in larger battles. The biggest down sides are that it has no long-range firepower at all, and that it's on the low end of the Medium chart, meaning less physical damage and lower maximum armor and structure limitations.
The standard PHX-1 likewise makes use of a pair of MGs to supplement its laser firepower without further overextending its heat, but that model is not in the poll, so I chose the well-armored -1K in spite of its lack of jump capability. As for the -1D, jumping is great when you need it, but raises the to-hit numbers for your own shots just as much as it raises the opponents' shots at you, so it's not something I do often except to get out of bad situations, and the -1D lacks the MGs of the -1 or -1K; a good variant, but shy of greatness.
My third choice would have been the HBK-4J, rather than the -4G or -4P listed. Packing the LRM compliment of a Whitworth or Dervish into its 50 ton frame while still mounting 5 of the 8 MLs of the -4P means that it's equally at home and dangerous as a fire-support design or as a brawler, and makes a perfect bodyguard unit for a fire support lance until the LRMs run out, at which point it advances to wreak havoc with the MLs in full energy zombie mode. Incidentally, while a DV is slightly "better" than a WTH in a couple of respects, the WTH is superior to it per ton, and far cheaper for roughly comparable utility if cost is considered in a campaign. Again, it's not in the poll.