Depends somewhat on what moveset you're planning to use on Dragonair/Dragonite eventually, but I'd look at getting either an Electric or Psychic-type; both are very useful for the E4.
For Electric, Ampharos or Magneton is probably best.
Ampharos has a slightly better movepool (Signal Beam and Power Gem, though they come fairly late, plus Focus Blast from a TM if you want), is bulkier, and easier to train.
Magneton's Steel typing is nice to have at times and it's a bit faster. If you can trade with DPPt to get a Magnezone, it's also stronger.
For Psychic, consider Alakazam, Jynx, Exeggutor, Espeon, maybe Starmie or Slowbro if you're willing to double down on the Water typing a bit.
Alakazam is a monster offensively, but requires trading to get and is fragile. It also doesn't have a hugely useful movepool beyond Psychic (though TMs can round it out nicely with stuff like Shadow Ball, Focus Blast, Grass Knot, or Shock Wave), but Alakazam's raw power and awesome speed make it the best overall choice. Even without trading, Kadabra is pretty good in a pinch.
Jynx is really good and somewhat underrated. If you don't have an Ice move on Vaporeon or Dragonair it will help a lot with Lance etc. However, its level up moves are pretty bad and mostly use its terrible physical attack, requiring you to use TMs to unlock its real potential. Since the Psychic TM only comes after the Elite 4, it might not be the best choice.
Exeggutor allows you to still have a Grass type and is quite powerful, but it can't get a better Psychic move than Confusion without a TM unless you raise Exeggcute to 47 before evolving, which is a lot to ask.
Starmie is fast, tougher than you think, and has a lot of great movepool options. However, it also really needs TMs to be most effective, Psychic in particular. Surf, Psychic + pick two of Recover/Thunder(bolt)/Blizzard or Ice Beam makes it nasty.
Slowbro is slow (duh), but very bulky. It shares a need for the Psychic TM for maximum usefulness unless you're willing to wait until 50+. However, Slowbro does have the option of running a Curse set and going for the slow, tanky, physical attacking route. Something like Curse + Zen Headbutt along with two of Avalanche, Surf, Slack Off can be very strong and tough to kill, and is readily available before the Elite 4.