I am going to go over what heroes are top tier roamers and what heroes can be left out. At the moment I think that Alistar is the game’s strongest roamer. Alistar is superior to other roamers because he has 2 abilities in a gank, and when his flash is up he can use pulverize/headbutt and if used correctly will most always result in a kill or a save.
Not only is this combination deadly early game, once he gets 6 Alistar can dive towers with his ultimate up and take literally no damage. He does not need to have farm to be useful late game. This allows him to buy wards all game with oracles and roam around with basically no items. Because of his ultimate, he is nearly impossible to gank and it is really hard to take oracles off of Alistar.
(Side note: Alistar’s flash is the most deadly flash in the game, and should only be used only for assuring a kill/save. An Alistar roaming without flash cooldown up is not only very gankable, but is much harder to secure kills/saves with, so save flash for key moments).
The next roamers are basically preference, Janna is very strong because with only 1 point invested into Q she has a full on knockup, which can interrupt and be used as a ministun on a group of players. Also, she has a built in speed buff/slow combination on W, and shield on E. Finally, her ultimate is another spell that is a double ability, a super knockback in case one of your carries gets caught in a bad position + a minor AOE heal (Janna shouldn’t really be farmed enough to make her heal too noticable, and her ult is usually just used as another knockback). Taric has a nice ranged stun with a lot of base armor with his W ability, and a pretty negligible heal. Taric’s abilities are inferior to Janna’s because all he offers is a stun, which can be arguably better or worse than Janna’s Q. However, Janna’s ult absolutely destroys Tarics ult in terms of being a very clutch ability. Taric is very beefy, and can survive longer when being beat on which is an interesting trade off, less utility + more survivability. Also, Taric’s stun is much easier to land, making him a much easier hero to play than Janna.
Finally, Blitzcrank is also a very strong roam, and his hook can be devastating in early game ganks, and late game ganks. Blitzcrank is overall a great roamer as he offers not only a nasty grab on a fairly short cooldown, he can stun once he lands the grab and usually can make the enemy flash out or they are dead. Not to mention his passive gives some insane survivability. These are all “Tier 1″ roamers, and are very strong candidates for bans/first picks in ranked play.
Next is the “Tier 2″ roamers. These roamers offer much less both early game and late game than “Tier 1″ roamers, and need more experience and farm to be viable late game, which is taking farm away from your carries. The first “Tier 2″ roamer I want to talk about is Eve. An Eve forces mid and top to play very defensively and can scout buffs and help the jungler gank the enemy jungler very well. Eve is probably the strongest early game roam, but brings absolutely nothing to a teamfight which is why I put her as tier 2. To counter Eve, buy pink wards. But more importantly, when there is an Eve on the other team you can force early game 5 mans on dragon, towers, and Baron. Eve does absoultely nothing in a standard 5v5 teamfight, and is extremely easy to focus.
Other “Tier 2″ roamers (or supports) who lack in certain areas include Lux, Karma, Kayle, Garen, Soraka, Sona, and Poppy. I hesitate to put Poppy into a “Tier 2″ category because she can be very viable early and late game. From my experience, however, Poppy players tend to soak XP too much and rely on early game CS before they begin ganking. Overall, however, these tier 2 roamers basically have no place in a solo lane, no place in a roaming role because they lack either serious survivability or have absoultely no lane pressence. Soraka is a great example of a hero that is not viable against a skilled comp of players because she:
A. Is way too squishy
B. Has absolutely 0 CC
C. Basically does not fit any role in the current metagame, as supports that need farm in order to contribute to teamfights -ARE NO LONGER VIABLE-.
Euro comps with Soraka/X in lane at bot are going to get beat every single time by a solo tank because no hero can put out enough damage to force a good tank out of lane, and Soraka does not pressure lanes. Also, if Soraka/X in lane at bot is going to push the tower, they will be forced to be way out of position and extremely overextended. At this point, the bot tank calls for a gank and the roamer and jungle come bot and completely destroy Soraka/X and take the tower.
As for the rest of the tier 2 roamers, I feel they have no place in ranked Solo queue and from my experience will usually burden the team at some point in the game, with exceptions always. Also, because I have much more experience with solo queue than tournament play, I cannot say that these tier 2 roamers are not viable in tournament play. However, the comp MUST be built around them, and I still feel that they are inferior to tier 1 roamers.